Fix the CHANGELOG.rst
Fix the CHANGELOG.rst

Fixes the amount of dashes under the title for the changelog

mmetak/streamlink,melmorabity/streamlink,javiercantero/streamlink,gravyboat/streamlink,back-to/streamlink,back-to/streamlink,chhe/streamlink,chhe/streamlink,wlerin/streamlink,javiercantero/streamlink,mmetak/streamlink,melmorabity/streamlink,streamlink/streamlink,bastimeyer/streamlink,wlerin/streamlink,beardypig/streamlink,gravyboat/streamlink,streamlink/streamlink,beardypig/streamlink,bastimeyer/streamlink