fix(learn): correct mocking example

[cjihrig]

Description

This example appears to be incorrect.

Validation

Related Issues

Fixes: nodejs/node#58462

Check List

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• I have run pnpm format to ensure the code follows the style guide.
• I have run pnpm test to check if all tests are passing.
• I have run pnpm build to check if the website builds without errors.
• I've covered new added functionality with unit tests if necessary.

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Name	Status	Preview	Updated (UTC)
nodejs-org	✅ Ready (Inspect)	Visit Preview	May 26, 2025 4:17pm

[Copilot]

Pull Request Overview

This PR fixes an incorrect mock API usage in the example provided in mocking.md.

• Corrects the method chaining on the mock function in the code snippet.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.35%. Comparing base (d261c9f) to head (987cc53).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7779      +/-   ##
==========================================
+ Coverage   75.34%   75.35%   +0.01%     
==========================================
  Files          96       96              
  Lines        7861     7861              
  Branches      192      192              
==========================================
+ Hits         5923     5924       +1     
+ Misses       1937     1936       -1     
  Partials        1        1              

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[github-actions]

Lighthouse Results

URL	Performance	Accessibility	Best Practices	SEO	Report
/en	🟠 77	🟢 100	🟢 100	🟢 91	🔗
/en/about	🟢 100	🟢 100	🟢 100	🟠 82	🔗
/en/about/previous-releases	🟢 100	🟢 100	🟢 100	🟠 83	🔗
/en/download	🟢 98	🟢 100	🟢 100	🟢 91	🔗
/en/blog	🟢 100	🟢 100	🟢 96	🟢 92	🔗