Fix docs on field customization

[ricardogsilva]

This PR fixes the docs section that describes field customization by correcting the erroneous code samples

---

• fixes Bug: custom field docs don't match code implementation #736

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.72%. Comparing base (2540c51) to head (07ff879).
⚠️ Report is 60 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #742      +/-   ##
===========================================
- Coverage   100.00%   99.72%   -0.28%     
===========================================
  Files           75       86      +11     
  Lines         5817     6850    +1033     
===========================================
+ Hits          5817     6831    +1014     
- Misses           0       19      +19     

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[jowilf]

Thanks @ricardogsilva