docs(config): warn about spaces in schedule

[HonkingGoose]

Changes

• Warn users that in the schedule config option:
  • 5am is correct
  • 5 am (with space) is wrong

Context

@rarkins fixed a repository configuration error, they were using a space between 4 and am:

• Action Required: Fix Renovate Configuration slsa-framework/slsa-github-generator#404 (comment)
• chore(renovate): use cron syntax for schedule slsa-framework/slsa-github-generator#3638

The schedule docs only show examples of the correct format, but it's easy to assume that 4 am will work. Or to accidentally add a space, because that feels natural.

I think we should warn louder about this possible gotcha. 😄

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[rarkins]

Could we massage this instead?

[viceice]

Could we massage this instead?

I'm not sure, it can probably break some other scenarios

[renovate-release]

🎉 This PR is included in version 37.368.3 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 37.368.3

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