[css-values-4] Editorial. Fix type map for <number> values.

Author: tabatkins

css-values-4/Overview.bs

@@ -3990,7 +3990,8 @@ Serialization</h3>
 			2. Serialize the keyword ''infinity'', ''-infinity'', or ''NaN'',
 				as appropriate to represent the value,
 				and append it to |s|.
-			3. If |fn|’s [=CSSNumericValue/type=] is anything other than «[ "number" → 1 ]»,
+			3. If |fn|’s [=CSSNumericValue/type=] is anything other than «[ ]»
+				(empty, representing a <<number>>),
 				append " * " to |s|.
 				Create a numeric value
 				in the [=canonical unit=] for |fn|'s [=CSSNumericValue/type=]