[css-flexbox-1] fix typo

Author: vwkd

css-flexbox-1/Overview.bs

@@ -307,7 +307,7 @@ Overview</h3>
 		Here's an example of a catalog where each item has a title, a photo, a description, and a purchase button.
 		The designer's intention is that each entry has the same overall size,
 		that the photo be above the text,
-		and that the purchase buttons aligned at the bottom, regardless of the length of the item's description.
+		and that the purchase buttons are aligned at the bottom, regardless of the length of the item's description.
 		Flex layout makes many aspects of this design easy:
 
 		* The catalog uses flex layout to lay out rows of items horizontally,