Update go get buf.build in README.md

The old import path stopped working. 

```
go get go.buf.build/protocolbuffers/go/prometheus/prometheus
go: unrecognized import path "go.buf.build/protocolbuffers/go/prometheus/prometheus": https fetch: Get "https://go.buf.build/protocolbuffers/go/prometheus/prometheus?go-get=1": dial tcp: lookup go.buf.build on 192.168.2.1:53: no such host
```

We should instead tell users to use the new import paths. 

Signed-off-by: Matthias Loibl <mail@matthiasloibl.com>

README.md

@@ -149,7 +149,7 @@ We are publishing our Remote Write protobuf independently at
 You can use that as a library:
 
 ```shell
-go get go.buf.build/protocolbuffers/go/prometheus/prometheus
+go get buf.build/gen/go/prometheus/prometheus/protocolbuffers/go@latest
 ```
 
 This is experimental.