fixing wording

source/core/zone-sharding.txt

@@ -114,15 +114,15 @@ You must use fields contained in the :term:`shard key` when defining a new
 range for a zone to cover. If using a :term:`compound <compound index>` shard
 key, the range must include the prefix of the shard key.
 
-For example, given a shard key ``{ a : 1, b : 2, c : 3 }``, creating or
-updating a zone to cover values of ``b`` requires including ``a`` as the
-prefix. Creating or updating a zone to covers values of ``c`` requires
+For example, given a shard key ``{ a : 1, b : 1, c : 1 }``, creating or
+updating a range to cover values of ``b`` requires including ``a`` as the
+prefix. Creating or updating a range to covers values of ``c`` requires
 including ``a`` and ``b`` as the prefix.
 
-You cannot create zones using fields not included in the shard key. For
+You cannot create ranges using fields not included in the shard key. For
 example, if you wanted to use zones to partition data based on
-geographic location, the shard key would need at least one field that
-contained geographic data.
+geographic location, the shard key would need the first field to
+contain geographic data.
 
 When choosing a shard key for a collection, consider what fields you might
 want to use for configuring zones. After sharding, you cannot change the