repo-lockdown: Add pr comment and fix typo

Author: tstellar

.github/workflows/repo-lockdown.yml

@@ -8,8 +8,11 @@ permissions:
 
 jobs:
   action:
-    runs-on: ubuntu-lastest
+    runs-on: ubuntu-latest
     steps:
       - uses: dessant/repo-lockdown@v2
         with:
           process-only: 'prs'
+          pr-comment: >
+            This repository does not accept pull requests.
+            Please follow http://llvm.org/docs/Contributing.html#how-to-submit-a-patch for contribution to LLVM.