[css-values-5] allow a single syntax component. Closes #11088

Author: tabatkins

css-values-5/Overview.bs

@@ -213,7 +213,7 @@ used in specifications to define CSS features,
 and which represents a [=syntax definition=]:
 
 <pre class="prod def" nohighlight>
-	<dfn><<syntax>></dfn> = '*' | <<syntax-component>> [ <<syntax-combinator>> <<syntax-component>> ]+ | <<syntax-string>>
+	<dfn><<syntax>></dfn> = '*' | <<syntax-component>> [ <<syntax-combinator>> <<syntax-component>> ]* | <<syntax-string>>
 	<dfn><<syntax-component>></dfn> = <<syntax-single-component>> <<syntax-multiplier>>?
 	                   | '<' transform-list '>'
 	<dfn><<syntax-single-component>></dfn> = '<' <<syntax-type-name>> '>' | <<ident>>