docs: Fix example service registration for built-in proxy

Fix the sample service registration for the built-in proxy by adding
the missing `sidecar_service` block.

ui/packages/consul-ui/mock-api/v1/agent/services

@@ -12,6 +12,7 @@
     "ModifyIndex": 0,
     "ProxyDestination": "",
     "Connect": {
+      "SidecarService": {
         "Proxy": {
           "ExecMode": "daemon",
           "Command": [
@@ -22,6 +23,7 @@
           "Config": null
         }
       }
+    }
   },
   "web-proxy": {
     "Kind": "connect-proxy",

website/content/docs/connect/proxies/built-in.mdx

@@ -24,6 +24,7 @@ for the built-in proxy.
   "service": {
     "name": "example-service",
     "connect": {
+      "sidecar_service": {
         "proxy": {
           "config": {
             "bind_address": "0.0.0.0",
@@ -45,6 +46,7 @@ for the built-in proxy.
       }
     }
   }
+}
 ```
 
 All fields are optional with a reasonable default.