Fix Exposed statement in dfn of Sanitizer interface (#218)

Identifiers of an extended attribute need to be enclosed in parentheses.

See: Web IDL: https://webidl.spec.whatwg.org/#dfn-extended-attribute

index.bs

@@ -221,7 +221,7 @@ page to query and predict what effect a given configuration will have, or
 to build a new configuration based on an existing one.
 
 <pre class=idl>
-[Exposed=Window,Worker]
+[Exposed=(Window,Worker)]
 interface Sanitizer {
   constructor(optional SanitizerConfig config = {});
   SanitizerConfig get();