Point to rubygems instead of Rails GitHub. [ci skip]

The rails repository is not the official plugin anymore

guides/source/security.md

@@ -841,7 +841,7 @@ It is recommended to _use RedCloth in combination with a whitelist input filter_
 
 NOTE: _The same security precautions have to be taken for Ajax actions as for "normal" ones. There is at least one exception, however: The output has to be escaped in the controller already, if the action doesn't render a view._
 
-If you use the [in_place_editor plugin](https://github.com/rails/in_place_editing), or actions that return a string, rather than rendering a view, _you have to escape the return value in the action_. Otherwise, if the return value contains a XSS string, the malicious code will be executed upon return to the browser. Escape any input value using the h() method.
+If you use the [in_place_editor plugin](https://rubygems.org/gems/in_place_editing), or actions that return a string, rather than rendering a view, _you have to escape the return value in the action_. Otherwise, if the return value contains a XSS string, the malicious code will be executed upon return to the browser. Escape any input value using the h() method.
 
 ### Command Line Injection