[YAML-tmLanguage] Remove empty capture group from multi-line match rule

[radium-v]

Fixes the rule for matching match, begin, and end multi-line blocks.

[keith-hall]

Thanks for your contribution. I'd be more confident about approving this if we had some syntax tests for YAML-tmLanguage... I will try to find time to try this change out locally within the next few days. Someone may beat me to it and that's fine 😉