Fixes AppendVecError::SizeMismatch error message

[brooksprumo]

Problem

PR #6552 added a new AppendVecError variant, but the error message has the wrong variable indexes 🤦.

Summary of Changes

Fix 'em.

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.

[HaoranYi]

lgtm.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.3%. Comparing base (0f647dd) to head (2771096).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6792   +/-   ##
=======================================
  Coverage    83.3%    83.3%           
=======================================
  Files         853      853           
  Lines      378185   378185           
=======================================
+ Hits       315296   315351   +55     
+ Misses      62889    62834   -55     

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