[vtadmin] Do not backtick binary name

[ajm188]

Description

I accidentally stumbled over the behavior outlined in flag.PrintDefaults 1, which pflag replicates, specifically:

The listed type, here int, can be changed by placing a back-quoted
name in the flag's usage string; the first such item in the message is
taken to be a parameter name to show in the message and the back quotes
are stripped from the message when displayed.

Related Issue(s)

rohit-nayak-ps/vreplication-docs#2 (review)

Checklist

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

Deployment Notes

[vitess-bot]

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