Update the instant-merge.yaml file definition to comply yamllint check

Author: jstourac

.github/workflows/instant-merge.yaml

@@ -1,6 +1,7 @@
+---
 name: Instant Merge Konflux PRs
 
-on:
+on:  # yamllint disable-line rule:truthy
   pull_request:
     types:
       - opened
@@ -23,4 +24,4 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ github.token }}
         run: |
-          gh pr merge --merge --admin ${{ github.event.pull_request.html_url }}
+          gh pr merge --merge --admin ${{ github.event.pull_request.html_url }}