Fix formatting and errors in std::getopts example.

There were three issues effecting the example in the getopts rustdoc:

1. The blockquote was incorrectly formatted.  Fixed by switching to using
   an explicit markdown code section with ```.
2. The `fail fail_str(f)` would not compile.  Fixed by using `fail!()` instead
   of `fail`.
3. The line `matches.free[0]` produced a compile error about moving from
   an immutable vector.  Fix by using `copy`.