Fix build_doc.yml

Checkout `refs/pull/$PR/head` instead of `refs/pull/$PR/merge`.

If the pull-request has conflicts, that is different.

.github/workflows/build_doc.yml

@@ -65,7 +65,7 @@ jobs:
         if: steps.get_round.outputs.result != 'stop'
         with:
            repository: ${{ github.repository }}
-           ref: refs/pull/${{ steps.get_pr_number.outputs.result }}/merge
+           ref: refs/pull/${{ steps.get_pr_number.outputs.result }}/head
            fetch-depth: 2
 
       - name: install dependencies