docs: link to map blog post

docs/schematypes.pug

@@ -450,7 +450,7 @@ block content
 
     _New in Mongoose 5.1.0_
 
-    A `MongooseMap` is a subclass of the built-in [`Map` class](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map).
+    A `MongooseMap` is a subclass of [JavaScript's `Map` class](http://thecodebarbarian.com/the-80-20-guide-to-maps-in-javascript.html).
     In these docs, we'll use the terms 'map' and `MongooseMap` interchangeably.
     In Mongoose, maps are how you create a nested document with arbitrary keys.