[doc] correct pip command to have needed dependancies to use the start command I know we want to remove this command but in the meantime having this documentation working seems like something good.

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
https://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))