[css-color-5] Fix doubled paren in syntax of hsl(), fix w3c#7362

Author: svgeesus

css-color-5/Overview.bs

@@ -1124,7 +1124,7 @@ Relative color syntax is only applicable to the non-legacy RGB syntactic forms.
 The grammar of the ''hsl()'' function is extended as follows:
 
 <pre class='prod'>
-<dfn>hsl()</dfn> = hsl(([from <<color>>]?
+<dfn>hsl()</dfn> = hsl([from <<color>>]?
 			  [<<hue>> | none]
 			  [<<percentage>> | <<number>> | none]
 			  [<<percentage>> | <<number>> | none]