As it says in the docs:
You may create your own templates for controller actions (show, edit and history), in other case default built-in templates will be used.
So, in your views
folder, create a folder called wiki_pages
and then put your new templates in that folder.
Here's what's going on:
When your WikiPagesController currently goes to render a wiki page, it looks for a template in apps/views/wiki_page
corresponding to the current action. That folder/file doesn't exist, so it looks in other directories and ultimately finds the template in the gem. (You should be able to see this process in your console.)
When you create the folder and add the template (as above), the WikiPagesController finds the template in your application and renders that, instead of rendering the template provided by the gem.