Fix broken links in README.md setup section

[lbeaufort]

So that our setup documentation is correct, we need to update our README.md with the correct links under "Project prerequisites" #2 and #3.

"2. Set up your Node environment— learn how to do this with our Javascript Ecosystem Guide." should link here:
https://github.com/18F/dev-environment-standardization/blob/18f-pages/pages/languages/javascript.md

"3. Set up your Python environment— learn how to do this with our Python Ecosystem Guide." should link here:
https://github.com/18F/dev-environment-standardization/blob/18f-pages/pages/languages/python.md