Explicitly set jwt_audience in README example

Ought to make it more clear that the JWT audience is something you
need to keep track off.

README.md

@@ -22,6 +22,7 @@ jobs:
         uses: andreaso/[email protected]
         with:
           vault_server: https://vault.example.com:8200
+          jwt_audience: vault.example.com
           oidc_backend_path: github-oidc
           oidc_role: example-user
           ssh_backend_path: ssh-client-ca