Error in documentation on python develop env about collatex

[djbpitt]

@jorisvanzundert Thank you for the correction! I've changed the cd collatex/ line to cd collatex/collatex-pythonport. It happened too late for today's release of version 2.2, but it will be in the next one.

With respect to the paragraph at the end, though, I think it should be possible to check out a branch from anywhere within the repo, and not only from the main directory. There is a risk of problems if you check out a branch from a subdirectory that has been deleted within the branch you're checking out, but that seems like a fringe situation. I'll close the issue, but please feel free to reopen it if there's something about the checkout directory that I've overlooked.