Fix typo for --cells flag help description in ApplyRoutingRules

[iheanyi]

Description

While reading through the documentation for ApplyRoutingRules, I noticed this typo in the flag. This pull request fixes that.

Related Issue(s)

N/A

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

N/A

[vitess-bot]

Review Checklist

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General

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[mattlord]

Thanks, @iheanyi !