docs: minor improvements to the service discovery README.md (#5296)

i) Increased the size of the Service Discovery Readme title
ii) Changed `TargetGroups` to "target groups" as it has been relocated and renamed to another package.

Signed-off-by: tariqibrahim <tariq181290@gmail.com>

discovery/README.md

@@ -1,4 +1,4 @@
-### Service Discovery
+# Service Discovery
 
 This directory contains the service discovery (SD) component of Prometheus.
 
@@ -131,23 +131,23 @@ the Prometheus server will be able to see them.
 
 ### The SD interface
 
-A Service Discovery (SD) mechanism has to discover targets and provide them to Prometheus. We expect similar targets to be grouped together, in the form of a [`TargetGroup`](https://godoc.org/github.com/prometheus/prometheus/config#TargetGroup). The SD mechanism sends the targets down to prometheus as list of `TargetGroups`.
+A Service Discovery (SD) mechanism has to discover targets and provide them to Prometheus. We expect similar targets to be grouped together, in the form of a [target group](https://godoc.org/github.com/prometheus/prometheus/discovery/targetgroup#Group). The SD mechanism sends the targets down to prometheus as list of target groups.
 
 An SD mechanism has to implement the `Discoverer` Interface:
 ```go
 type Discoverer interface {
-	Run(ctx context.Context, up chan<- []*config.TargetGroup)
+	Run(ctx context.Context, up chan<- []*targetgroup.Group)
 }
 ```
 
-Prometheus will call the `Run()` method on a provider to initialize the discovery mechanism. The mechanism will then send *all* the `TargetGroup`s into the channel. 
-Now the mechanism will watch for changes. For each update it can send all `TargetGroup`s, or only changed and new `TargetGroup`s, down the channel. `Manager` will handle 
+Prometheus will call the `Run()` method on a provider to initialize the discovery mechanism. The mechanism will then send *all* the target groups into the channel. 
+Now the mechanism will watch for changes. For each update it can send all target groups, or only changed and new target groups, down the channel. `Manager` will handle 
 both cases.
 
 For example if we had a discovery mechanism and it retrieves the following groups:
 
 ```
-[]config.TargetGroup{
+[]targetgroup.Group{
   {
     Targets: []model.LabelSet{
        {
@@ -187,11 +187,11 @@ For example if we had a discovery mechanism and it retrieves the following group
 }
 ```
 
-Here there are two `TargetGroups` one group with source `file1` and another with `file2`. The grouping is implementation specific and could even be one target per group. But, one has to make sure every target group sent by an SD instance should have a `Source` which is unique across all the `TargetGroup`s of that SD instance. 
+Here there are two target groups one group with source `file1` and another with `file2`. The grouping is implementation specific and could even be one target per group. But, one has to make sure every target group sent by an SD instance should have a `Source` which is unique across all the target groups of that SD instance. 
 
-In this case, both the `TargetGroup`s are sent down the channel the first time `Run()` is called. Now, for an update, we need to send the whole _changed_ `TargetGroup` down the channel. i.e, if the target with `hostname: demo-postgres-2` goes away, we send:
+In this case, both the target groups are sent down the channel the first time `Run()` is called. Now, for an update, we need to send the whole _changed_ target group down the channel. i.e, if the target with `hostname: demo-postgres-2` goes away, we send:
 ```
-&config.TargetGroup{
+&targetgroup.Group{
   Targets: []model.LabelSet{
      {
         "__instance__": "10.11.122.11:6001",
@@ -209,7 +209,7 @@ down the channel.
 
 If all the targets in a group go away, we need to send the target groups with empty `Targets` down the channel. i.e, if all targets with `job: postgres` go away, we send:
 ```
-&config.TargetGroup{
+&targetgroup.Group{
   Targets: nil,
   "Source": "file2", 
 }