Use main branch as comparison point

The default is master and the action does not use the merge/checkout
information to determine what is modified.

Author: jonahgraham

.github/workflows/check-markdown.yaml

@@ -12,3 +12,4 @@ jobs:
     - uses: gaurav-nelson/github-action-markdown-link-check@v1
       with:
         check-modified-files-only: 'yes'
+        base-branch: 'main'