update example workflow in README

[spencerlepine]

👋🏻 Hi there!

This template will help you describe the work you’ve done, so one of the project owners can review and potentially accept it promptly. Please fill relevant sections below and feel free to delete what does not apply to your work (including this comment).

Thank you for helping us improve Image Actions! ⚡️

---

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.

Related issues

Are there any related issues or feature requests this work will resolve? Please mention them here. E.g.:

n/a

Learn more about linking Pull Requests to issues here.

Screenshots

n/a

Reviewers

Mention one or more project owners here or request review directly in the right sidebar in the Reviewers section:

• @benschwarz for engineering review
• @thefoxis for content and copy review

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[benschwarz]

@spencerlepine thank you