docs: use codex plugin add for plugin installation (#151)

Author: twishabansal

README.md

@@ -170,7 +170,7 @@ codex plugin marketplace add GoogleCloudPlatform/data-agent-kit
 **2. Install the plugin:**
 
 ```bash
-codex plugin install looker@data-agent-kit
+codex plugin add looker@data-agent-kit
 ```
 
 **3. Set env vars:**