Updating comment for Proxy::New function

The comment for the Proxy::New functions seems inaccurate and is
currently identical to Map::New:

3090   /**
3091    * Creates a new empty Map.
3092    */
3093   static Local<Map> New(Isolate* isolate);

This commit updates the comment to describe that it creates a Proxy and
not a Map.

BUG=

Review-Url: https://codereview.chromium.org/2176063002
Cr-Commit-Position: refs/heads/master@{#39002}

include/v8.h

@@ -3627,7 +3627,7 @@ class V8_EXPORT Proxy : public Object {
   void Revoke();
 
   /**
-   * Creates a new empty Map.
+   * Creates a new Proxy for the target object.
    */
   static MaybeLocal<Proxy> New(Local<Context> context,
                                Local<Object> local_target,