[Github Action] Update cron to run main branch sync every 10 min (#2933)

Author: hanzhan1

.github/workflows/sync-main.yml

@@ -2,7 +2,7 @@ name: automatic sync main branch from llvm-project to llvm
 
 on:
   schedule:
-  - cron: '/10 * * * *'
+  - cron: '*/10 * * * *'
 jobs:
   sync:
     runs-on: ubuntu-latest