docs: Correct google_firestore_backup_schedule recurrence descriptions

[rwhogg]

There was some inaccurate documentation for google_firestore_backup_schedule

Release Note Template for Downstream PRs (will be copied)

[github-actions]

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@hao-nan-li, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

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[hao-nan-li]

/gcbrun

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))
google-beta provider: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))

[modular-magician]

Tests analytics

Total tests: 19
Passed tests: 19
Skipped tests: 0
Affected tests: 0

Click here to see the affected service packages

• firestore

$\textcolor{green}{\textsf{All tests passed!}}$
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