[css-namespaces-3] Wrap definition of @namespace in <dfn> (w3c#9402)

Author: tidoust

css-namespaces-3/Overview.bs

@@ -89,7 +89,7 @@ Terminology</h3>
 
 <h2 id="declaration">Declaring namespaces: the ''@namespace'' rule</h2>
 
-	The ''@namespace'' <a>at-rule</a> declares a namespace prefix
+	The <dfn>@namespace</dfn> <a>at-rule</a> declares a namespace prefix
 	and associates it with a given namespace name (a string).
 	This namespace prefix can then be used in namespace-qualified names
 	such as the <a>CSS qualified names</a> defined below.