Update readme link to `Pip requirement file format`

[zerocewl]

The link in the readme to Pip requirement file format seems to be outdated.

This page has moved

You should be redirected automatically in 3 seconds. If that didn’t work, here’s a link: pip install

I guess it should point to his one?

https://pip.pypa.io/en/stable/reference/requirements-file-format/