Fix MCPServerEntry example field accuracy

- Fix headerForward: use addPlaintextHeaders map instead of
  nonexistent headers list field
- Fix externalAuthConfigRef: remove kind field that doesn't exist
  on ExternalAuthConfigRef (only AuthServerRef has kind)

Both verified against v0.17.0 source code.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisJBurns

docs/toolhive/guides-vmcp/configuration.mdx

@@ -125,19 +125,17 @@ spec:
   # Optional: token exchange for authenticating to the remote server
   externalAuthConfigRef:
     name: my-auth-config
-    kind: MCPExternalAuthConfig
 
   # Optional: custom CA bundle for TLS verification
   caBundleRef:
     configMapRef:
       name: internal-ca-bundle
       key: ca.crt
 
-  # Optional: inject custom headers into requests
+  # Optional: inject plaintext headers into requests
   headerForward:
-    headers:
-      - name: X-Custom-Header
-        value: my-value
+    addPlaintextHeaders:
+      X-Custom-Header: my-value
 ```
 
 **When to use MCPServerEntry vs. MCPRemoteProxy:**