[css-align] wording

Author: tabatkins

css-align/Overview.bs

@@ -486,7 +486,7 @@ Overflow Alignment: the ''safe'' and ''unsafe'' keywords</h3>
 Content Distribution: the 'justify-content' and 'align-content' properties</h2>
 
 	The <dfn export>content distribution properties</dfn> 'justify-content' and 'align-content'
-	control alignment of the box's content within the box.
+	control alignment of the box's content within its content box.
 
 	<div class="figure">
 		<img alt="Diagram showing that the alignment of the content within the element is affected." src="images/content-example.svg" width=212 height=212>