[css-flexbox] Fix #436

Author: fantasai

css-flexbox/Overview.bs

@@ -682,7 +682,7 @@ Absolutely-Positioned Flex Children</h3>
 
 	<div class='example'>
 		The effect of this is that if you set, for example,
-		''align-content: center;'' on an absolutely-positioned child of a <a>flex container</a>,
+		''align-self: center;'' on an absolutely-positioned child of a <a>flex container</a>,
 		auto offsets on the child will center it in the <a>flex container's</a> <a>cross axis</a>.
 
 		However, since the absolutely-positioned box is considered to be “fixed-size”,