[css-pseudo] Export first formatted line.

Author: fantasai

css-pseudo/Overview.bs

@@ -140,7 +140,7 @@ Finding the First Formatted Line</h4>
 
   In CSS, the ''::first-line'' pseudo-element
   can only have an effect when attached to a <i>block container</i>.
-  The <dfn>first formatted line</dfn> of an element
+  The <dfn export>first formatted line</dfn> of an element
   must occur inside a block-level descendant in the same flow
   (i.e., a block-level descendant that is not out-of-flow due to floating or positioning).