Fix errors in contributing.rst for setup

[JasonWeill]

Fix errors in setup docs: removes a redundant pip command, and updates the repository's punctuation.

[github-actions]

👈 Launch a Binder on branch jupyterlab/jupyter-collaboration/JasonWeill-patch-1

[davidbrochart]

removes a redundant pip command

Why is it redundant?

[JasonWeill]

Line 32 mentions that users need to install both node and pip before starting with installation. Line 34 starts, "Once you have installed the dependencies above …". Either we need to delete line 32 and start the instructions with installing both node and pip, or the documentation needs to presume, at the start of the code block, that the user has already installed both node and pip. The previous version of this file included instructions to install pip, but not node.

[davidbrochart]

But pip install --upgrade pip upgrades pip, right?

[JasonWeill]

It does; is that required? As long as the user has supported versions of pip and node, they should be OK to start with the command after the "upgrade pip" command.

[jtpio]

Thanks!