Fix documentation to the custom Deprecator

[ci skip]

activesupport/lib/active_support/core_ext/module/deprecation.rb

@@ -15,7 +15,7 @@ class Module
   #
   #   class MyLib::Deprecator
   #     def deprecation_warning(deprecated_method_name, message, caller_backtrace)
-  #        message = "#{method_name} is deprecated and will be removed from MyLibrary | #{message}"
+  #        message = "#{deprecated_method_name} is deprecated and will be removed from MyLibrary | #{message}"
   #        Kernel.warn message
   #     end
   #   end