Add missing labels to namelabelmutator.antrea.io in doc (#7122)

The `namelabelmutator.antrea.io` MutatingWebhookConfiguration YAML in
antrea-network-policy.md is missing the necessary labels to ensure that
the CA cert is synced correctly by the Antrea Controller (e.g., when the
certificate is rotated).

Signed-off-by: Antonin Bas <[email protected]>

Author: antoninbas

docs/antrea-network-policy.md

@@ -1226,6 +1226,10 @@ kind: MutatingWebhookConfiguration
 metadata:
   # Do not edit this name.
   name: "labelsmutator.antrea.io"
+  # Do not remove these labels.
+  labels:
+    app: antrea
+    served-by: antrea-controller
 webhooks:
   - name: "namelabelmutator.antrea.io"
     clientConfig: