[css-color-5][editorial] device-cmky also serialized

Author: svgeesus

css-color-5/Overview.bs

@@ -2346,7 +2346,9 @@ or any other color or monochrome output device which has been characterized.
 This section extends [[css-color-4#serializing-color-values]]
 to add serialization of the
 results of the
-''color-mix()'', and
+''color-mix()'', 
+''device-cmyk()'',
+and
 relative color functions.
 
 In this section, the strings used in the specification and the corresponding characters are as follows.