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

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](https://github.com/airbnb/ruby/blob/dfdc84b89284d537043b7eb4aa3ccc607940303f/rubocop-airbnb/spec/rubocop/cop/airbnb/module_method_in_wrong_file_spec.rb#L96) 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](https://github.com/airbnb/ruby/blob/dfdc84b89284d537043b7eb4aa3ccc607940303f/rubocop-airbnb/lib/rubocop/cop/airbnb/module_method_in_wrong_file.rb#L88)) which returns `Foo::Foo` instead of just `Foo`.

Start with rubocop-airbnb/lib/rubocop/cop/airbnb/module_method_in_wrong_file.rb around line 88 and the spec at rubocop-airbnb/spec/rubocop/cop/airbnb/module_method_in_wrong_file_spec.rb around line 96. Run that spec and inspect normalize_module_name(node.parent_module_name). Done means the singleton-class case reports “Method baz should be defined in foo.rb” and the spec checks the offense text.

Written by the indexing model from the issue text.