Update the version Bump workflow to mention how to run GH actions

Author: Himani Anil Deshpande

.github/workflows/bump_version.yml

@@ -39,6 +39,7 @@ jobs:
         title: 'Bump version to ${{ inputs.pcluster-version }}'
         body: |
           This PR contains version bump.
+          Please close and re-open the PR for Github Actions to run.
           Auto-generated by Github Action
         branch: versionbump${{ inputs.branch }}${{ inputs.pcluster-version }}
         delete-branch: true