Change github actions trigger to use main branch (facebook#769)

Summary:
This hasn't been updated since we renamed from `master`, and resulted in deploy not triggering.

Pull Request resolved: facebook#769

Reviewed By: motiz88

Differential Revision: D34106356

Pulled By: rh389

fbshipit-source-id: ad880d0d95653ec366265c2616ef920770f85732

.github/workflows/deploy-website.yml

@@ -3,7 +3,7 @@ name: Deploy
 on:
   push:
     branches:
-      - "master"
+      - "main"
     paths:
       - "docs/**"
       - "website/**"