docs: updates docs example to use actions/checkout@v3 instead of v2

[yjose]

Right now, if you copy-paste a workflow example and run it in your repo, you will get the following warning:

calibreapp/image-actions
Node.js 12 actions are deprecated. Please update the following actions to use Node.js 16

Updating the checkout action to version 3 will fix the issue, as it updates the default runtime to Node.js 16.

[CLAassistant]

All committers have signed the CLA.

[benschwarz]

Thanks @yjose 🙏