Incorrect documentation for `restore-keys`

[Zac-HD]

https://github.com/actions/cache/blob/04f198bf0b2a39f7230a4304bf07747a0bddf146/README.md#L52

This turns out to be slightly but importantly wrong: the format is actually a multi-line string with one key per line; attempting to supply an actual list of keys will fail to validate (c.f. https://github.com/python/cpython/pull/104468#pullrequestreview-1425935982). I think the just updating the docs is probably better than switching to the currently-documented behavior.

[github-actions[bot]]

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[Zac-HD]

https://github.com/actions/cache/tree/main#inputs shows the same problem; the lack of a fix doesn't imply the issue should be closed.