fixed commenting bot not working

Fix the mentioned issue: paritytech/command-bot#113 (comment)

.github/workflows/command-inform.yml

@@ -7,9 +7,9 @@ on:
 jobs:
   comment:
     runs-on: ubuntu-latest
+    if: contains(github.event.comment.html_url, '/pull/') && startsWith(github.event.comment.body, 'bot ')
     steps:
     - name: Inform that the new command exist
-      if: ${{ github.event.issue.pull_request && startsWith(github.event.comment.body, 'bot ') }}
       run: gh pr comment ${{ github.event.issue.number }} --body 'We are migrating this bot to be a GitHub Action<br/><br/>Please, see the <a href="https://github.com/paritytech/polkadot-sdk/blob/master/.github/commands-readme.md">documentation on how to use it</a>'
       env:
         GH_TOKEN: ${{ github.token }}