docs(datetime): remove outdated mention of utc option

[yibe]

Fixes a trivial oversight in the docs that was missed in 4ec7dd4 (#5647)

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.33%. Comparing base (d02d714) to head (68e9cbb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6301      +/-   ##
==========================================
- Coverage   96.34%   96.33%   -0.01%     
==========================================
  Files         547      547              
  Lines       41667    41667              
  Branches     6314     6314              
==========================================
- Hits        40144    40141       -3     
- Misses       1481     1484       +3     
  Partials       42       42              

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[kt3k]

LGTM. Good catch! Thanks