docs: Fix wxt-vitest-plugin reference

[emmanuel-ferdman]

PR Summary:
PR #646 moved the location of the wxt-vitest-plugin.ts file. This PR adjusts sources to changes.

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[aklinker1]

Ah, thanks. Looks like that's the only reference that wasn't updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.72%. Comparing base (a5667f8) to head (e1427e4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #650      +/-   ##
==========================================
- Coverage   86.98%   86.72%   -0.27%     
==========================================
  Files         114      114              
  Lines        9215     9215              
  Branches      928      921       -7     
==========================================
- Hits         8016     7992      -24     
- Misses       1185     1209      +24     
  Partials       14       14              

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