Give a better example of val use cases

[GarethFlandro]

The existing variables and descriptions were things that would be updated as soon as the next customer came, whereas the price would likely remain the same.

[sarahhaggarty]

Hi @GarethFlandro
Thanks for your suggestion! I discussed it with our developer advocates and after some thought we decided to leave the code as-is. The price itself can also change. The purpose of the example is to demonstrate what we recommend in the note: that by default, variables should be declared as read-only and made mutable only if necessary.
I'll close the PR now but thanks for sharing your thoughts with us!