[css-writing-modes-3] fix typo

Author: fantasai

css-writing-modes-3/Overview.bs

@@ -2043,7 +2043,7 @@ Auto-sizing Orthogonal Flow Roots</h4>
   (i.e. the size used when its [=preferred size property=] is ''width/auto'')
   is calculating as its [=fit-content size=], i.e.
   <code>min([=max-content inline size=], max([=min-content inline size=], [=stretch-fit inline size=])</code>,
-  where the [=available space=] use to calculate the [=stretch-fit inline size=] is
+  where the [=available space=] used to calculate the [=stretch-fit inline size=] is
   either the size of the [=containing block=] if that is [=definite=],
   or else the [=fallback size=] as defined [[#orthogonal-auto|above]].