docs: add event parameter to build status badge
docs: add event parameter to build status badge

This feature hasn't been deployed to shields.io yet and doesn't do
anything yet, but will hopefully be available soon. Adding the parameter
now doesn't break it.

streamlink/streamlink-twitch-gui,chhe/livestreamer-twitch-gui,chhe/livestreamer-twitch-gui,streamlink/streamlink-twitch-gui,chhe/livestreamer-twitch-gui,streamlink/streamlink-twitch-gui