[css-2015] Fix typo

Author: simevidas

css-2015/Overview.bs

@@ -533,7 +533,7 @@ Requirements for Responsible Implementation of CSS</h2>
 				where only the prefixed syntax is supported
 				greatly reduces the risk of stylesheets
 				targetting only this prefixed syntax being written.
-				This in turns allow UA vendors to retire
+				This in turn allows UA vendors to retire
 				their prefixed syntax once the feature is stable,
 				without risking breaking existing content.
 				It also reduces the need occasionally felt by by some vendors