[css-shapes-1][editorial] Export 'reference box', with an appropriate for= value.

Author: tabatkins

css-shapes-1/Overview.bs

@@ -316,7 +316,7 @@ Basic Shapes</h2>
 	can be specified using basic shape functions.
 	When using this syntax
 	to define shapes,
-	the <dfn>reference box</dfn> is defined
+	the <dfn for="<basic-shape>" export>reference box</dfn> is defined
 	by each property that uses
 	<<basic-shape>> values.
 	The coordinate system for the shape