Error in uninstallation instructions.

[Reid Burton]

I was in the process of freeing up space on my computer, and I found that
cygwin was taking up more space than I thought, I was trying to uninstall
it, but it was failing to delete /dev/nul, the problem was that dos systems
are forbidden from deleting files with that and a few other names (see
https://superuser.com/questions/282194/how-do-i-remove-a-file-named-nul-on-windows)
I believe that this information should be included in the removal
documentation.