Fix typo in docstring for send_alert parameters
Fix typo in docstring for send_alert parameters

We're missing a colon. See https://pysensu-yelp.readthedocs.org/en/latest/#pysensu_yelp.send_event
Yelp/pysensu-yelp,aclevy/pysensu-yelp,Yelp/pysensu-yelp,aclevy/pysensu-yelp