Update documentation for shadowenv exec

[robinbrandt]

(Don't know if this is a regression or a bug in the docs. This would be to improve the docs if it's the latter...)

This change updates the documentation to be more explicit about where to put command and arguments for shadowenv exec (and fixing some typos).

The previous documentation suggested that program arguments need to be specified after a --. But it's actually both the command name and the arguments.

The old text:

$ shadowenv exec --help
...
USAGE:
    shadowenv exec [OPTIONS] <child-argv0|child-argv> [child-argv0] [-- <child-argv>...]

ARGS:
    <child-argv0>      If the command takes no arguments, it can be passed directly as the last arugment.
    <child-argv>...    If the command requires arguments, they must all be passed after a --.

This is in contrast of how it actually works.

Bad:

$ shadowenv exec ls -- /
error: The argument '<child-argv>...' cannot be used with one or more of
  the other specified arguments

Good:

$ shadowenv exec -- ls /
Applications    cores           etc ...

[burke]

Oh yeah good catch, thanks!