Apply suggestion from @drammock

Author: drammock

action.yaml

@@ -23,4 +23,4 @@ runs:
       shell: bash
       run: >-
         python -m pip install pyyaml requests;
-        python ${{ github.action_path }}/sync-teams-to-gh.py -o ${{ github.repository_owner }} -p ${YAML_PATH:=teams.yaml} -m >> $GITHUB_STEP_SUMMARY
+        python ${{ github.action_path }}/sync-teams-to-gh.py -o ${{ github.repository_owner }} -p ${YAML_PATH:-teams.yaml} -m >> $GITHUB_STEP_SUMMARY