Fix contribution tutorial errors

[drvinceknight]

Pointed out by @katiemcgoldrick:

• The source command was not recognised, and I have to instead run
  env\Scripts\activate.
• The make command also wasn't recognised, so I ran sphinx-build -b html . _build/html instead and the documentation built as expected

Nashpy/docs/contributing/reference/contributors/main.rst does not exist, it should be Nashpy/docs/contributing/reference/contributors/index.rst