Update README.py to reflect Netbox code change

[Kage1]

Per Netbox readme: https://github.com/netbox-community/netbox/releases/v3.5.0

• "The obj context variable is no longer passed when rendering custom links: Use object instead."

The obj. reference in the README was updated to object.

The code snippet is working in our Netbox instance.

[TheNetworkGuy]

Great find! I'll merge your request :)