In programming guide, `numTasks` is used in several places as arguments of Transformations. However, in code, `numPartitions` is used. In this fix, I replace `numTasks` with `numPartitions` in programming guide for consistency.

Author: Cheng Wang <chengwang0511@gmail.com>

Closes #18774 from polarke/replace-numtasks-with-numpartitions-in-doc.