docs: use singular agy plugin install

Correct the Antigravity CLI command from `plugins install` to `plugin install`.

Author: twishabansal

README.md

@@ -134,7 +134,7 @@ You can install plugins directly from a remote GitHub repository.
 **1. Install the plugin:**
 
 ```bash
-agy plugins install https://github.com/gemini-cli-extensions/cloud-sql-mysql
+agy plugin install https://github.com/gemini-cli-extensions/cloud-sql-mysql
 ```
 
 **2. Set env vars:**