fixing typo s/uti/until

Author: Andrew Leung

source/administration/sharding.txt

@@ -490,7 +490,7 @@ In such cases, data import to an empty MongoDB shard cluster can
 become slower than to a replica set. The reason for this is how
 MongoDB writes data and manage chunks in a shard cluster.
 
-MongoDB inserts data into a chunk unti it becomes large and splits on
+MongoDB inserts data into a chunk until it becomes large and splits on
 the same shard. If the balancer notices a chunk imbalance between
 shards, a migration process will begin to evenly distribute chunks.