docs: Update uninstall command to reflect multiple extension support (#13582)

Author: JayadityaGit

docs/extensions/index.md

@@ -54,10 +54,11 @@ gemini extensions install <source> [--ref <ref>] [--auto-update] [--pre-release]
 
 ### Uninstalling an extension
 
-To uninstall, run `gemini extensions uninstall <name>`:
+To uninstall one or more extensions, run
+`gemini extensions uninstall <name...>`:
 
 ```
-gemini extensions uninstall gemini-cli-security
+gemini extensions uninstall gemini-cli-security gemini-cli-another-extension
 ```
 
 ### Disabling an extension