controllers: HTTPVerbController: Update docstring

This commit updates the docstring of the `HTTPVerbController` class to
reflect the change in the way HEAD requests are handled by Milla.

src/milla/controllers.py

@@ -104,15 +104,8 @@ class HTTPVerbController(Controller):
             def GET(self, request):
                 return 'Hello, world!'
 
-            HEAD = GET
-
             def POST(self, request):
                 return 'Thanks!'
-
-    This example also allows ``HEAD`` requests, by processing them as
-    ``GET`` requests. *Milla* handles this correctly, as it does not
-    send a response body for ``HEAD`` requests, even if the controller
-    callable returns one.
     '''
 
     def __call__(self, request, *args, **kwargs):