Add link to first mention of d3.map

For consistency with the previous mention of d3.set, which was linked.

CHANGES.md

@@ -207,7 +207,7 @@ var yields = [
 var sites = d3.set(yields, function(d) { return d.site; }); // Grand Rapids, Duluth, Waseca, Crookston, Morris
 ```
 
-The d3.map constructor also follows the standard array accessor argument pattern.
+The [d3.map](https://github.com/d3/d3-collection#map) constructor also follows the standard array accessor argument pattern.
 
 The *map*.forEach and *set*.forEach methods have been renamed to [*map*.each](https://github.com/d3/d3-collection#map_each) and [*set*.each](https://github.com/d3/d3-collection#set_each) respectively. The order of arguments for *map*.each has also been changed to *value*, *key* and *map*, while the order of arguments for *set*.each is now *value*, *value* and *set*. This is closer to ES6 [*map*.forEach](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/forEach) and [*set*.forEach](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set/forEach). Also like ES6 Map and Set, *map*.set and *set*.add now return the current collection (rather than the added value) to facilitate method chaining. New [*map*.clear](https://github.com/d3/d3-collection#map_clear) and [*set*.clear](https://github.com/d3/d3-collection#set_clear) methods can be used to empty collections.