ci: fix check_flaws shallow clone missing merge base

Add fetch-depth: 0 to the checkout step in docs_flaw_checker.yml.
Without a full clone, tj-actions/changed-files cannot find the merge
base between main and the PR branch, causing the job to fail with
"no merge base".

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/docs_flaw_checker.yml

@@ -17,6 +17,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0
 
       - name: Install Node.js
         uses: actions/setup-node@v3