update example workflow in README

[spencerlepine]

👋🏻 Hi there!

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Thank you for helping us improve Image Actions! ⚡️

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What does this PR introduce?

In a few bullet points, please describe the changes this Pull Request makes. E.g.:

• Updated the example workflow in README, so it will run as-is / out-of-the-box

The workflow needs write access to update the pull request. Also updated to use latest checkout workflow
ref: https://stackoverflow.com/questions/70435286/resource-not-accessible-by-integration-on-github-post-repos-owner-repo-ac

->> Committing files…
	 *  Head SHA: 21d1d205dc11c152fec6d38f8ffaf8de1bdbffc4
	 *  Tree 6d1450717ae764a8226f0672ee4b2942f2b22961
	 *  Converting images to blobs…
(node:1) UnhandledPromiseRejectionWarning: HttpError: Resource not accessible by integration
    at /usr/local/src/image-actions/node_modules/@octokit/request/lib/request.js:56:27
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
(node:1) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:1) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

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[benschwarz]

@spencerlepine thank you