Editorial: Fix type assertion in BubbleRelativeDuration

[ptomato]

This operation can be called with a time unit as largestUnit. It should continue to work as advertised, the type assertion in the header was just wrong.

Closes: #3121

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.85%. Comparing base (2512852) to head (281a3c2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3123   +/-   ##
=======================================
  Coverage   96.85%   96.85%           
=======================================
  Files          21       21           
  Lines        9983     9983           
  Branches     1829     1829           
=======================================
  Hits         9669     9669           
  Misses        268      268           
  Partials       46       46           

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