fixed doc

The contribution is my original work. I license the work to the project under the project's open source license.

Small typo in the programming guide.

Author: Eric Chiang <eric.chiang.m@gmail.com>

Closes #5599 from ericchiang/docs-typo and squashes the following commits:

1177942 [Eric Chiang] fixed doc

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.