Fix missing flag usage for vault credentials flags

[mattlord]

Description

This is a follow-up to #11336 where I somehow missed replacing the flag usage with the pflag flag set for the vault related flags.

Related Issue(s)

• Follow-up to: Move dbconfigs to pflag and remove deprecated flags #11336

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests are not required
• Documentation is not required

[vitess-bot]

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