Update event.md
Update event.md

Changing "a" to "an".
"receive an HTTP POST"
I suppose this change could be open for debate, depending on how you pronounce "HTTP"
katieporter/docs,ksigler7/docs,Zaftula/docs,jacobmovingfwd/docs,sendgrid/docs,Whatthefoxsays/docs,hardikjain29/docs,Zaftula/docs,jacobmovingfwd/docs,Zaftula/docs,Aonepathan/docs,Zaftula/docs,hardikjain29/docs,ksigler7/docs,Aonepathan/docs,Zaftula/docs,jacobmovingfwd/docs,katieporter/docs,Aonepathan/docs,sendgrid/docs,hardikjain29/docs,jacobmovingfwd/docs,Aonepathan/docs,Whatthefoxsays/docs,Whatthefoxsays/docs,jacobmovingfwd/docs,Aonepathan/docs,hardikjain29/docs,katieporter/docs,hardikjain29/docs,katieporter/docs,ksigler7/docs,sendgrid/docs,katieporter/docs