Update defaul branch for github action
Update defaul branch for github action

Summary: Changing the branch triggering github action CI, as default branch changed from master to main.

Reviewed By: stuclar

Differential Revision: D31035490

fbshipit-source-id: 57bcb582ae4c3b4615c801ec295b2ea46c46b494

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter