docs: replace old parameter on example code (#346)

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Hi maintainers, thank you for the awesome project.

I think that `outputs.url` seems to be deprecated parameter because it's
not included in README.md[^1] now and it returns same value of
`outputs.version_url`[^2].
so I replaced `outputs.url` to `outputs.version_url` on example code on
README.md

[^1]:
https://github.com/google-github-actions/deploy-appengine/blob/abf67aa4ea465aee02104003e9f377d8ec976ade/README.md#outputs
[^2]:
https://github.com/google-github-actions/deploy-appengine/blob/abf67aa4ea465aee02104003e9f377d8ec976ade/src/main.ts#L248-L253

README.md

@@ -41,7 +41,7 @@ jobs:
 
     # Example of using the output
     - id: 'test'
-      run: 'curl "${{ steps.deploy.outputs.url }}"'
+      run: 'curl "${{ steps.deploy.outputs.version_url }}"'
 ```
 
 ## Inputs