docs: Fix variable name in custom-matcher-api code example

[tohjustin]

Summary

This PR fixes #8581.

Test plan

Tested changes made to the docs by:

1. Building the Jest website locally (following commands outlined in CONTRIBUTING.md)
2. Navigating to http://localhost:3001/docs/en/expect#thisutils
3. Inspecting changes made to the code example

[codecov-io]

Codecov Report

Merging #8582 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #8582   +/-   ##
=======================================
  Coverage   63.24%   63.24%           
=======================================
  Files         272      272           
  Lines       11311    11311           
  Branches     2760     2760           
=======================================
  Hits         7154     7154           
  Misses       3540     3540           
  Partials      617      617

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[thymikee]

Thanks!

[tohjustin]

Thanks for the quick review! 😅

[github-actions]

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