Corrected instructions for exercise_01

[JHertz5]

Thanks for this repository, I'm working through the exercises now and finding it useful. I struggled a bit with the step in exercise_01

• Modify run.py to add a tb_lib library and add all source files in test/ to that library

Because the code snippet

tb_lib = prj.add_library("tb_lib")
tb_lib.add_source_files(join(root, "test", "*.vhd"))

contains apparent mistakes:

• The Python variables in the supplied script are uppercase, but the snippet uses lowercase. Python is case sensitive, so copying this into run.py doesn't work.

• The snippet supplied does not use the join method correctly (maybe this is from an earlier version of python? Not sure).

I've suggested some changes to help future users of the repo. For HDL users who are not familiar with Python these could stop them from learning how to use VUnit.