docs: use claude official marketplace for installation

Replace the manual '/plugin marketplace add' + install steps with a
single '/plugin install cloud-sql-mysql@claude-plugins-official' step, matching
the approach used in cloud-sql-postgresql.

Author: twishabansal

README.md

@@ -166,16 +166,10 @@ 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-mysql.git#0.2.0
-```
-
-**4. Install the plugin:**
-
-```bash
-/plugin install cloud-sql-mysql@cloud-sql-mysql-marketplace
+/plugin install cloud-sql-mysql@claude-plugins-official
 ```
 
 _(Tip: Run `/plugin list` inside Claude Code to verify the plugin is active, or `/reload-plugins` if you just installed it.)_