ci: docpreview: fix body-includes parameter

Otherwise finding the existing comment fails.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

.github/workflows/docpreview.yml

@@ -58,7 +58,7 @@ jobs:
         with:
           issue-number: ${{ github.event.pull_request.number }}
           comment-author: 'github-actions[bot]'
-          body-includes: documentation preview
+          body-includes: preview for this PR
 
       - name: Create or update comment
         uses: peter-evans/create-or-update-comment@v4