[css-cascade] fix english

Author: tabatkins

css-cascade/Overview.bs

@@ -747,7 +747,7 @@ Cascading</h2>
 <h3 id='cascading-origins'>
 Cascading Origins</h3>
 
-	Each style rule has an <dfn id="origin" export local-lt="origin">cascade origin</dfn>,
+	Each style rule has a <dfn id="origin" export local-lt="origin">cascade origin</dfn>,
 	which determines where it enters the cascade.
 	CSS defines three core <a>origins</a>: