[css-contain-1] remove stray text

See #5806

Author: frivoal

css-contain-1/Overview.bs

@@ -87,7 +87,7 @@ Strong Containment: the 'contain' property</h2>
 		the shortcut values (''strict'' and ''content''),
 		instead of computing to themselves,
 		compute to the corresponding keywords.
-		Given than the effect on is identical,
+		Given than the effect is identical,
 		this allows implementations not to store the precise syntax through which this was achived.
 		Also, thanks to the shortest-serialization princple,
 		this makes this unimportant difference non observable via serialization.