docs(README): remove github-api-url from most basic example (actions#120)

Author: gr2m

README.md

@@ -30,7 +30,6 @@ jobs:
         with:
           app-id: ${{ vars.APP_ID }}
           private-key: ${{ secrets.PRIVATE_KEY }}
-          github-api-url: "https://github.acme-inc.com/api/v3"
       - uses: ./actions/staging-tests
         with:
           token: ${{ steps.app-token.outputs.token }}