docs: use claude official marketplace to install

Author: twishabansal

README.md

@@ -126,19 +126,14 @@ In your terminal, set your environment vars as described in the [configuration s
 claude
 ```
 
-**3. Add the marketplace:**
+**3. Install the plugin:**
 
 ```bash
-/plugin marketplace add https://github.com/gemini-cli-extensions/cloud-sql-postgresql.git#0.4.0
-```
-
-**4. Install the plugin:**
-
-```bash
-/plugin install cloud-sql-postgresql@cloud-sql-postgresql-marketplace
+/plugin install cloud-sql-postgresql@claude-plugins-official
 ```
 
 _(Tip: Run `/plugin list` inside Claude Code to verify the plugin is active, or `/reload-plugins` if you just installed it.)_
+In case you do not see the plugin, update your marketplace.
 
 </details>