Use actual path in fish mkdir function

Before this update the actual problem was in:

    > cd /tmp/
    > mkdir foo
    fish: The function calls itself immediately, which would result in an infinite loop.
    /Users/nfedyashev/.config/fish/functions/mkdir.fish (line 2): mkdir -p $argv
                                                                  ^
    in function 'mkdir',
    	called on standard input,
    	with parameter list 'foo'

Fix is based on fish-shell author's <a href="https://github.com/fish-shell/fish-shell/issues/224#issuecomment-6922707">comment</a>