Fix C const comment

[nobu]

Fix RDoc::Parser::C#handle_constants.

• Backslashes here are eaten at parse time, these gsubs do nothing.
• The content literal in TestRDocParserC#test_do_constants is not using single quotes, so the backslashes are eaten at parse time too and the above bug has not been tested.
• Limit colon separating the new definition to the first word-ending colon, not to split strings including colons, e.g., URLs and Windows path names.