[documentation] update config.river sample for k8s ebpf use case (#3128)

* [readme] fix documentation to prevent load errors

Error: /etc/agent/config.river:61:47: field "targets" does not exist

60 |     forward_to = [pyroscope.write.endpoint.receiver]
61 |     targets = discovery.relabel.specific_pods.targets
   |                                               ^^^^^^^
62 | }

* Fix another error in pyrsocope_write

URL needs to be part of a endpoint structure.

---------

Co-authored-by: Christian Simon <simon@swine.de>

examples/grafana-agent-auto-instrumentation/ebpf/k8s/config.river

@@ -59,10 +59,12 @@ pyroscope.ebpf "instance" {
 }
 
 pyroscope.write "endpoint" {
-    url = "http://pyroscope:4040"
-    // url = "<Grafana Cloud URL>"
-    // basic_auth {
-    //  username = "<Grafana Cloud User>"
-    //  password = "<Grafana Cloud Password>"
-    // }
+    endpoint {
+      url = "http://pyroscope:4040"
+      // url = "<Grafana Cloud URL>"
+      // basic_auth {
+      //  username = "<Grafana Cloud User>"
+      //  password = "<Grafana Cloud Password>"
+      // }
+    }
 }