What this PR does:

Fixes serverImage example in README to have `@sha256` when referencing an image ID.

Which issue(s) this PR fixes:
N/A

Checklist

- [x] Changes manually tested
- [ ] Automated Tests added/updated
- [x] Documentation added/updated
- [ ] CHANGELOG.md updated (not required for documentation PRs)
- [ ] CLA Signed: DataStax CLA

docs/user/README.md

@@ -429,7 +429,7 @@ metadata:
 spec:
   serverType: cassandra
   serverVersion: 3.11.7
-  serverImage: private-docker-registry.example.com/cass-img/cassandra-with-mgmtapi:1a2b3c4d
+  serverImage: private-docker-registry.example.com/cass-img/cassandra-with-mgmtapi@sha256:1a2b3c4d
 ```
 
 DSE:
@@ -441,7 +441,7 @@ metadata:
 spec:
   serverType: dse
   serverVersion: 6.8.4
-  serverImage: private-docker-registry.example.com/dse-img/dse:5f6e7d8c
+  serverImage: private-docker-registry.example.com/dse-img/dse@sha256:5f6e7d8c
 ```
 
 ## Configuring a NodePort service