docs: use claude official marketplace for installation (gemini-cli-extensions#171)

Author: twishabansal

README.md

@@ -126,20 +126,13 @@ 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
+/plugin install cloud-sql-postgresql@claude-plugins-official
 ```
 
-**4. Install the plugin:**
-
-```bash
-/plugin install cloud-sql-postgresql@cloud-sql-postgresql-marketplace
-```
-
-_(Tip: Run `/plugin list` inside Claude Code to verify the plugin is active, or `/reload-plugins` if you just installed it.)_
-
+_(Tip: Run `/plugin list` inside Claude Code to verify the plugin is active, or `/reload-plugins` if you just installed it.)
 </details>
 
 <details>