fix typo in core/sharding-internals.txt

Author: jrassi

source/core/sharding-internals.txt

@@ -222,7 +222,7 @@ Furthermore:
   throughout the cluster, then the failure of the entire shard will
   render the entire cluster unavailable.
 
-In essence, this concern for reliably simply underscores the
+In essence, this concern for reliability simply underscores the
 importance of choosing a shard key that isolates query operations to a
 single shard.