Use "$PWD" instead of `pwd` in install instructions

There's no need to promote the use of the `pwd` command when shells
export the current directory as `$PWD`. Using `$PWD` makes the
commandline work in fish as well. Additionally, quote the variable so it
works properly even if the cwd has a space in it.
auto merge of #1100 : kballard/cargo/tweak-readme-install-instructions, r=alexcrichton

There's no need to promote the use of the `pwd` command when shells export the
current directory as `$PWD`. Using `$PWD` makes the commandline work in fish
as well. Additionally, quote the variable so it works properly even if the cwd
has a space in it.