ci(bug): Fix pr-merge workflow

Apparently, commit 3892f3b broke
the external PR merge when the contributor is introduced in the
yml file. This patch fixes it.

PR: #345
Signed-off-by: Anastassios Nanos <ananos@nubificus.co.uk>
Reviewed-by: Charalampos Mainas <cmainas@nubificus.co.uk>
Approved-by: Charalampos Mainas <cmainas@nubificus.co.uk>

Author: ananos

.github/workflows/pr-merge.yml

@@ -32,7 +32,7 @@ jobs:
         uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
           fetch-depth: 0
-          ref: ${{ github.event.pull_request.base.sha }}
+          ref: ${{ github.event.pull_request.base.ref }}
 
       - name: Generate urunc-bot token
         id: generate-token