Merge pull request #18131 from y-yagi/fix_command_in_plugins_guide

`bin/rake` does not exist in plugin directory [ci skip]

guides/source/plugins.md

@@ -433,7 +433,7 @@ Once your README is solid, go through and add rdoc comments to all of the method
 Once your comments are good to go, navigate to your plugin directory and run:
 
 ```bash
-$ bin/rake rdoc
+$ bundle exec rake rdoc
 ```
 
 ### References