plugins.pixiv: fix doc typo thats -> that's
plugins.pixiv: fix doc typo thats -> that's

gravyboat/streamlink,wlerin/streamlink,streamlink/streamlink,beardypig/streamlink,chhe/streamlink,beardypig/streamlink,wlerin/streamlink,bastimeyer/streamlink,gravyboat/streamlink,melmorabity/streamlink,streamlink/streamlink,melmorabity/streamlink,chhe/streamlink,bastimeyer/streamlink