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

.github/workflows/build.yml

@@ -3,11 +3,11 @@ name: build
 on:
   push:
     branches:
-      - master
+      - main
       - github_action
   pull_request:
     branches:
-      - master
+      - main
 
 jobs:
   build: