Fix pip install instructions

[clevinson]

The gemfury url instructions in the readme are missing a colon (:) after the <TOKEN>, and as a result, the instructions in this readme were not working for me.

I found the solution by reading gemfury's documentation here.

This pr fixes the instructions in the README.md

[markusbkoch]

Thanks, @clevinson . We had been using the command without the colon since the beginning internally. Both work for me:

MarkusPersonal:~ markusbkoch$ pip3 uninstall cadCAD
Uninstalling cadCAD-0.2.4:
  Would remove:
    /usr/local/lib/python3.7/site-packages/cadCAD-0.2.4.dist-info/*
    /usr/local/lib/python3.7/site-packages/cadCAD/*
Proceed (y/n)? y
  Successfully uninstalled cadCAD-0.2.4


MarkusPersonal:~ markusbkoch$ pip3 install cadCAD --extra-index-url https://<TOKEN>@repo.fury.io/blockscience/
Looking in indexes: https://pypi.org/simple, https://repo.fury.io/blockscience/
Collecting cadCAD
  Downloading https://repo.fury.io/blockscience/-/ver_3hbPf/cadCAD-0.2.4-py3-none-any.whl
Installing collected packages: cadCAD
Successfully installed cadCAD-0.2.4


MarkusPersonal:~ markusbkoch$ pip3 uninstall cadCAD
Uninstalling cadCAD-0.2.4:
  Would remove:
    /usr/local/lib/python3.7/site-packages/cadCAD-0.2.4.dist-info/*
    /usr/local/lib/python3.7/site-packages/cadCAD/*
Proceed (y/n)? y
  Successfully uninstalled cadCAD-0.2.4


MarkusPersonal:~ markusbkoch$ pip3 install cadCAD --extra-index-url https://<TOKEN>:@repo.fury.io/blockscience/
Looking in indexes: https://pypi.org/simple, https://repo.fury.io/blockscience/
Collecting cadCAD
  Downloading https://repo.fury.io/blockscience/-/ver_3hbPf/cadCAD-0.2.4-py3-none-any.whl
Installing collected packages: cadCAD
Successfully installed cadCAD-0.2.4

Regardless, I do agree it makes sense to stick to what's in the gemfury documentation. @JEJodesty , any thoughts before we merge this?