Fix syntax error (#2925)

Found while authoring web-platform-tests/wpt#11979

Author: lukebjerring

css-regions-1/Overview.bs

@@ -1089,7 +1089,7 @@ The NamedFlow interface</h3>
 
 	<pre class="idl">
 		[Exposed=Window,
-		 MapClass(CSSOMString, NamedFlow)] interface NamedFlowMap {
+		 MapClass=(CSSOMString, NamedFlow)] interface NamedFlowMap {
 			NamedFlow? get(CSSOMString flowName);
 			boolean has(CSSOMString flowName);
 			NamedFlowMap set(CSSOMString flowName, NamedFlow flowValue);