[css-position-3][editorial] Export 'inset-modified containing block'.

Author: tabatkins

css-position-3/Overview.bs

@@ -663,7 +663,7 @@ Absolute (and Fixed) Positioning</h3>
 	the [=inset properties=] effectively reduce the [=containing block=]
 	into which it is sized and positioned
 	by the specified amounts.
-	The resulting rectangle is called the <dfn>inset-modified containing block</dfn>.
+	The resulting rectangle is called the <dfn export>inset-modified containing block</dfn>.
 
 	If only one [=inset property=] in a given axis is ''top/auto'',
 	it is set to zero.