Update Overview.bs

Updated the syntax of`alpha()` function to have the `<alpha>` value required

Author: dletorey

css-color-5/Overview.bs

@@ -1811,8 +1811,7 @@ Relative alpha colors refer to an origin color, and only change the alpha channe
 The grammar of the ''alpha()'' function, new in this level, is as follows:
 
 <pre class='prod'>
-<dfn>alpha()</dfn> = alpha([from <<color>>]
-					[ / [<<alpha-value>> | none] ]? )
+<dfn>alpha()</dfn> = alpha([from <<color>>] / [<<alpha-value>>])
 </pre>