Fix flag name typo from #16852

[timvaillancourt]

Description

This PR fixes a typo introduced in #16852 that causes the new --expected-primary flag to update the wrong value, --new-primary

This slipped through our internal testing as we're using RPCs from code, not via the CLI 🤦

Related Issue(s)

#16852

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[GrahamCampbell]

Maybe good to backport to v21 too?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.42%. Comparing base (f40e076) to head (34f0fa5).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16921      +/-   ##
==========================================
- Coverage   69.43%   69.42%   -0.02%     
==========================================
  Files        1570     1570              
  Lines      203812   203846      +34     
==========================================
+ Hits       141517   141519       +2     
- Misses      62295    62327      +32     

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[timvaillancourt]

Maybe good to backport to v21 too?

@GrahamCampbell yes, hoping to get this corrected for v21