[css-flexbox] Fix typo

Author: fantasai

css-flexbox/Overview.bs

@@ -682,7 +682,7 @@ Flex Item Margins and Paddings</h3>
 
 	Percentage margins and paddings on <a>flex items</a>
 	are always resolved against their own axis:
-	left and right margins resolve against the containing's block width,
+	left and right margins resolve against the containing block's width,
 	and top and bottom margins resolve against the containing block's height.
 	Unlike blocks, <a>block-axis</a> margins do not resolve against
 	the inline dimension of their containing block.