[css-color-5][editorial] correct grammar for device-cmyk

Author: svgeesus

css-color-5/Overview.bs

@@ -2319,7 +2319,7 @@ or any other color or monochrome output device which has been characterized.
 		<pre class='prod'>
 			<dfn>device-cmyk()</dfn> = <<legacy-device-cmyk-syntax>> | <<modern-device-cmyk-syntax>>
 			<dfn>&lt;legacy-device-cmyk-syntax></dfn> = device-cmyk( <<number>>#{4} ) 
-			<dfn>&lt;modern-device-cmyk-syntax></dfn> = device-cmyk( <<cmyk-component>>{4} [ / <<alpha-value>> | ''none'' ]? )
+			<dfn>&lt;modern-device-cmyk-syntax></dfn> = device-cmyk( <<cmyk-component>>{4} [ / [ <<alpha-value>> | ''none'' ] ]? )
 			<dfn>&lt;cmyk-component></dfn> = <<number>> | <<percentage>> | ''none''
 		</pre>