fixed doc

Author: ericchiang

docs/programming-guide.md

@@ -1093,7 +1093,7 @@ for details.
 ### Shuffle operations
 
 Certain operations within Spark trigger an event known as the shuffle. The shuffle is Spark's
-mechanism for re-distributing data so that is grouped differently across partitions. This typically
+mechanism for re-distributing data so that it's grouped differently across partitions. This typically
 involves copying data across executors and machines, making the shuffle a complex and
 costly operation.