Update checkout action to v4 tag in README.md

[maarten-blokker]

The example workflow in the readme uses a step actions/checkout with version v3, this version uses Node.js 16 which is deprecated, the following message added to the run:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3

• An example of a run with v3: https://github.com/maarten-blokker/helm/actions/runs/8711668277
• An example of a run after updating to v4: https://github.com/maarten-blokker/helm/actions/runs/8711740405/job/23896519184

[jhoblitt]

actions/checkout also needs to be bumped to v4 to fix the nodejs warning.