source in install/project

[shingtaklam1324]

in https://leanprover-community.github.io/install/project.html, there is a line

• If you have not logged in since you installed Lean and mathlib, then you may need to first type source ~/.profile or source ~/.bash_profile depending on your OS.

Someone just asked about this in the Xena Discord, about an error on Windows, which was bash: /c/Users/<their username>/.profile: No such file or directory. I told them to just close Git Bash and reopen it, which should do the same thing. Weirdly, if I do ls -a in Git Bash on my computer, I do see ~/.profile.

It might be worth mentioning explicitly what to do/expect on each OS, since right now it's expecting users to already be familiar with the command line. I would make a PR but it's been years since I last used Mac/Linux, and Windows behaviour seems to vary?