Editorial: Fix type assertion in BubbleRelativeDuration

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

Author: ptomato

spec/duration.html

@@ -1750,7 +1750,7 @@ <h1>
           _isoDateTime_: an ISO Date-Time Record,
           _timeZone_: an available time zone identifier or ~unset~,
           _calendar_: a calendar type,
-          _largestUnit_: a date unit,
+          _largestUnit_: a Temporal unit,
           _smallestUnit_: a date unit,
         ): either a normal completion containing an Internal Duration Record or a throw completion
       </h1>