[css-ui] Fix highligh typo

resolves #2142

Author: ericwilligers

css-ui-4/Overview.bs

@@ -2135,7 +2135,7 @@ or maybe all values should compute to ''user-select/text''.
 		however, UAs are encouraged to keep the visual selection consistent
 		with what would get copied to the clipboard when copying.
 		Copying text that does not appear to be selected, or vice versa,
-		is highligh confusing to users.</span>
+		is highly confusing to users.</span>
 
 		Attempting to start a selection in an element where 'user-select' is ''user-select/none'',
 		such as by clicking in it or starting a drag in it,