missed workflow update (#14)

Author: maikuru

.github/workflows/automerge.yml

@@ -11,7 +11,7 @@ jobs:
     name: version automerge
     steps:
       - id: downstream
-        uses: maikuru/downstream-version-automerge@master
+        uses: maikuru/downstream-version-automerge@main
         with:
           production-branch: 'main'
           development-branch: 'develop'