[css-overflow-4] Add missing [].

Fixes w3c#7582.

Author: dbaron

css-overflow-4/Overview.bs

@@ -274,7 +274,7 @@ Values have the following meanings:
 		The given string is treated as an independent paragraph
 		for bidi purposes.
 
-	<dt dfn-type=function><dfn>fade( <<length>> | <<percentage>> )</dfn>
+	<dt dfn-type=function><dfn>fade( [ <<length>> | <<percentage>> ] )</dfn>
 	<dd>
 		Clip inline content that overflows its line box.
 		Characters may be only partially rendered.