minor: adding cross refence

Author: Sam Kleinman

source/release-notes/2.4.txt

@@ -370,7 +370,7 @@ key. Consider the following properties when using a hashed shard key:
   chunks. By default, the initial number of chunks is equal to twice
   the number of shards at creation time. You can change the number of
   chunks created, using the ``numInitialChunks`` option, as in the
-  following invocation of ``shardCollection``:
+  following invocation of :dbcommand:`shardCollection`:
 
   .. code-block:: javascript