Update README.md (DataDog#21328)

We made two changes to the documentation page:
1 - in the Annotations v1 example, we were not using the container name for check, which is incorrect and it will not work. Replaced the postgresql.check_names for postgres.check_names to match the container name.
2. We added a note to guide customers to match their container name in the integration check_names. We are open to suggestion regarding the added note in the documentation.

Co-authored-by: traegerdog <122398611+traegerdog@users.noreply.github.com>
Co-authored-by: Eric Weaver <eweaver755@gmail.com>

Author: 3 people

postgres/README.md

@@ -282,6 +282,7 @@ Then, [instrument your application container that makes requests to Postgres][11
 To configure this check for an Agent running on Kubernetes:
 
 ##### Metric collection
+**Note**: The `<container-name>.check_names` must match the container name where the integration needs to collect metrics. In the example, we are using `postgres.check_names` because the container name is postgres.
 
 Set [Autodiscovery Integrations Templates][13] as pod annotations on your application container. Aside from this, templates can also be configured with [a file, a configmap, or a key-value store][14].
 
@@ -293,9 +294,9 @@ kind: Pod
 metadata:
   name: postgres
   annotations:
-    ad.datadoghq.com/postgresql.check_names: '["postgres"]'
-    ad.datadoghq.com/postgresql.init_configs: '[{}]'
-    ad.datadoghq.com/postgresql.instances: |
+    ad.datadoghq.com/postgres.check_names: '["postgres"]'
+    ad.datadoghq.com/postgres.init_configs: '[{}]'
+    ad.datadoghq.com/postgres.instances: |
       [
         {
           "host": "%%host%%",