Clarify the difference between subject and let

[Rachel Moser]

Suggestion made by programmurr, for Lesson 3:

Let and subject are explained, but the difference in using them under the 'describe' headers isn't really.
At a very superficial glance, they could be thought of as interchangeable.

I just grabbed this post on SO that helps explain the differences between them, but maybe there are better resources out there.
https://stackoverflow.com/questions/38437162/whats-the-difference-between-rspecs-subject-and-let-when-should-they-be-used