merge: minor

Author: Sam Kleinman

source/core/sharding.txt

@@ -247,7 +247,7 @@ Shard Keys
 .. todo:: link this section to <glossary:shard key>
 
 "Shard keys" refer to the :term:`field` that exists in every
-:term:`document` in a collection that that MongoDB uses to distribute
+:term:`document` in a collection that MongoDB uses to distribute
 documents among the :term:`shards <shard>`. Shard keys, like
 :term:`indexes <index>`, can be either a single field, or may be a
 compound key, consisting of multiple fields.