`RuboCop::Cop::Airbnb::ModuleMethodInWrongFile` returns incorrect error message when defining a method inside singleton class

[RenzoMinelli]

When developing PR #189, tests were changed to use new rspec-rubocop syntax which also checks that the message generated is correct. After doing this, the spec that checks that an error is returned when using "<<" static methods and a non-matching name failed with an incorrect message:

Method baz should be defined in foo/foo.rb

When the expected message was:

Method baz should be defined in foo.rb

After digging a little about it to understand what was happening, we realized that this incorrect message is also present on master – but there are no assertions respecting the offense's text, thus the test succeeds.
The issue seems to be caused by the method normalize_module_name(node.parent_module_name) (on module_method_in_wrong_file.rb#L88) which returns Foo::Foo instead of just Foo.