[ruby/rdoc] Fix parsing of rb_define_module_under

Fixes Ruby Bug #15819

https://github.com/ruby/rdoc/commit/94a052d833

lib/rdoc/parser/c.rb

@@ -318,7 +318,7 @@ class RDoc::Parser::C < RDoc::Parser
                 \s*"(?<module_name_1>\w+)"\s*
               \)
             |
-              _under\s*\( # rb_define_module_under(module_under, module_name_1)
+              _under\s*\( # rb_define_module_under(module_under, module_name_2)
                 \s*(?<module_under>\w+),
                 \s*"(?<module_name_2>\w+)"
               \s*\)