support Administrator in fish_prompt.fish

- fish, version 3.1.0

- http://cygwin.com/
```plain
> uname -a
CYGWIN_NT-10.0 3.1.4(0.340/5/3) 2020-02-19 08:49 x86_64 Cygwin
```

- fish_prompt.fish
```plain
    if contains -- $USER root toor Administrator
```