uptime: move help strings to markdown file

[miles170]

#4368

$ cargo run --features 'uptime' -- uptime -h
    Finished dev [unoptimized + debuginfo] target(s) in 0.13s
     Running `target/debug/coreutils uptime -h`
Display the current time, the length of time the system has been up,
the number of users on the system, and the average number of jobs
in the run queue over the last 1, 5 and 15 minutes.

Usage: target/debug/coreutils uptime [OPTION]...

Options:
  -s, --since    system up since
  -h, --help     Print help
  -V, --version  Print version

[cakebaker]

Thanks :)