Fix example manifest.yaml

For GitOps sync to work, it requires a namespace.
Otherwise, it complains about:
"an empty namespace may not be set when a resource name is provided"

doc/user/clusters/agent/index.md

@@ -337,6 +337,7 @@ apiVersion: apps/v1
 kind: Deployment
 metadata:
   name: nginx-deployment
+  namespace: gitlab-agent  # Can be any namespace managed by you that the agent has access to.
 spec:
   selector:
     matchLabels: