Update README.md

Rename method `authenticate_admin_user` to `authenticate` as recommended on [the Changelog]( https://github.com/varvet/godmin/blob/master/CHANGELOG.md#200---2019-12-06)

README.md

@@ -733,7 +733,7 @@ module Admin
     include Godmin::ApplicationController
     include Godmin::Authentication
 
-    def authenticate_admin_user
+    def authenticate
       authenticate_user!
     end