Clarify cost function vs. monetary cost in Hybrid Jobs with PennyLane notebook

**Is your feature request related to a problem? Please describe.**
In [this notebook](https://github.com/aws/amazon-braket-examples/blob/main/examples/hybrid_jobs/2_Using_PennyLane_with_Braket_Jobs/Using_PennyLane_with_Braket_Jobs.ipynb), the term "cost" without a modifier is used several times in back-to-back cells to describe different things: the cost function one is trying to minimize, and the actual cost in dollars to the customer.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
Need to explicitly call out which cost is being referred to in each plot, and in each call to the cost tracker. Change the axes for the evaluation of the cost function over iterations to say "cost function," and add a section header before the cost tracking calls to say something along the lines of "Tracking Customer Costs."