[css-overflow-3] Export term scrollbar gutter.

Author: fantasai

css-overflow-3/Overview.bs

@@ -736,7 +736,7 @@ Reserving space for the scrollbar: the 'scrollbar-gutter' property</h2>
 
 	The space between the inner border edge and the outer padding edge
 	which user agents may reserve to display the scrollbar
-	is called the <dfn>scrollbar gutter</dfn>.
+	is called the <dfn export>scrollbar gutter</dfn>.
 
 	The 'scrollbar-gutter' property gives control to the author
 	over the presence of <a>scrollbar gutters</a>