[css-flexbox] Fix markup error...

Author: fantasai

css-flexbox/Overview.bs

@@ -404,7 +404,7 @@ Flex Layout Box Model and Terminology</h2>
 
 	<p>
 		A <dfn export>flex container</dfn> is the box generated by an element with a
-		computed 'display' of 'flex' or 'inline-flex'.
+		computed 'display' of ''flex'' or ''inline-flex''.
 		In-flow children of a flex container are called <dfn export title="flex item" id="flex-item">flex items</dfn>
 		and are laid out using the flex layout model.