Readme: card_name --> card_label

[wwwindisch]

Hi Prabhakar,
Thanks for your project. It works well on Ubuntu 22.04. One thing I came across:

The var "card_name" seems to be wrong (at least it doesn't work). Correct would be "card_label":

sudo modprobe v4l2loopback video_nr=20 card_label="Virtual Camera 1"

However, I'm not quite sure, whether this is a distro-specific flavor. Therefore I opened an issue instead of a pull request.