Variable name letter is misleading

[robertdavidwest]

Hello, this is pretty minor but I thought it would be useful for future students. This variable name letter is misleading. It makes it sound like the job will iterate over a string, I think this could confuse people.

I propose changing the variable name to item.

https://github.com/udacity/nd027-c3-data-lakes-with-spark/blob/master/Debugging_And_Optimization/exercises/starter/practice_broadcast_joins/broadcast_example.py#L12