Revert "[css-namespaces-3] Define <namespace-prefix> with case-sensitive <custom-ident>" (w3c#11089)

Author: cdoublev

css-namespaces-3/Overview.bs

@@ -150,7 +150,7 @@ Syntax</h3>
 	<pre>
 		@namespace <<namespace-prefix>>? [ <<string>> | <<url>> ] ;
 
-		<dfn>&lt;namespace-prefix&gt;</dfn> = <<custom-ident>>
+		<dfn>&lt;namespace-prefix&gt;</dfn> = <<ident>>
 	</pre>
 
 	Any ''@namespace'' rules must follow all @charset and @import rules