[css-inline] Export root inline box

Author: fantasai

css-inline/Overview.bs

@@ -31,7 +31,7 @@ This module defines the CSS Inline Layout model,
 replacing and extending the model as defined in CSS2.1.
 It is very much a work-in-progress, and implementers should reference CSS2.1 for now.
 
-The <dfn>root inline box</dfn> is an anonymous inline box
+The <dfn export>root inline box</dfn> is an anonymous inline box
 which is automatically generated to hold
 all of the <a>inline-level</a> contents of a <a>block container</a>
 (if it has any).