update github actions default from master to main

Summary: Branches updated some time ago, update this to match

Reviewed By: zpao

Differential Revision: D32693531

fbshipit-source-id: 02da79e3fb5fcd3b204be4605521a151b977ce2c

build/fbcode_builder/getdeps.py

@@ -956,7 +956,7 @@ jobs:
         )
         parser.add_argument(
             "--main-branch",
-            default="master",
+            default="main",
             help="Main branch to trigger GitHub Action on",
         )