Update README.md

Author: ConfigCat

README.md

@@ -22,7 +22,7 @@ Documentation: https://github.com/configcat/feature-flag-reference-validator
        steps:
        - uses: actions/checkout@v1
        - name: ConfigCat Feature Flag Cleanup
-         uses: configcat/github-action-feature-flag-cleanup@master
+         uses: configcat/github-action-feature-flag-cleanup@1.0.0
          with:
            configcat-api-key: ${{ secrets.CONFIG_CAT_API_KEY }}
            fail-on-warnings: false