docs: use codex plugin add for plugin installation

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: twishabansal

README.md

@@ -186,7 +186,7 @@ codex plugin marketplace add GoogleCloudPlatform/data-agent-kit
 **2. Install the plugin:**
 
 ```bash
-codex plugin install cloud-sql-postgresql@data-agent-kit
+codex plugin add cloud-sql-postgresql@data-agent-kit
 ```
 
 **3. Set env vars:**