[css-display-4] export definition

Author: rachelandrew

css-display-4/Overview.bs

@@ -1154,7 +1154,7 @@ Reading Order: the 'reading-flow' property</h2>
 A <dfn export>reading flow container</dfn> is a flex or grid container
 with a valid 'reading-flow' value other than 'normal'.
 
-The rendering-defined sibling reading flow of a <a>reading flow container</a>
+The <dfn export>rendering-defined sibling reading flow</dfn> of a <a>reading flow container</a>
 is an ordered list of in-flow children of the container.
 All children must render to an element and are considered siblings in the reading flow container.
 The order is determined by the 'reading-flow' property.