[chores] Added blankline and removed main from workflow trigger branches

Author: atif09

.github/workflows/backport.yml

@@ -4,7 +4,6 @@ on:
   push:
     branches:
       - master
-      - main
   issue_comment:
     types: [created]
 
@@ -40,4 +39,5 @@ jobs:
       comment_body: ${{ github.event.comment.body }}
     secrets:
       app_id: ${{ secrets.OPENWISP_BOT_APP_ID }}
-      private_key: ${{ secrets.OPENWISP_BOT_PRIVATE_KEY }}
+      private_key: ${{ secrets.OPENWISP_BOT_PRIVATE_KEY }}
+