Fix typo

Author: frivoal

css-ui-4/Overview.bs

@@ -1100,7 +1100,7 @@ Insertion caret</h3>
 	are not intended to apply to such mechanisms
 	nor outside of [=editable elements=],
 	though user agents may choose to take them into account.
-	Whether to do so and to what extend is up to the user agent.
+	Whether to do so and to what extent is up to the user agent.
 
 	Note: The cursor image shown
 	when hovering over text when the 'cursor' property is ''cursor/auto'',