Update .env.local.example (#40839)

We use the variable "SANITY_REVALIDATE_SECRET" on the index page, but
the .env.local.example does not have this variable.

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## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
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- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
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## Documentation / Examples

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doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

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