Update README.md in order to uptate checkout to v3

[Danieldiazi]

Changed actions/checkout@v2 to actions/checkout@v3 (https://github.com/home-assistant/actions/pull/72)

[home-assistant[bot]]

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[home-assistant[bot]]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks :+1:

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[frenck]

The CI error/issue is unrelated to this PR. Going to merge it in.