documentation style: there is no "the restricted command"

I see "ssh restricted command" as a feature.  There is no command
provided; alternatively one could argue that the user is to provide a
script, which they will configure as the restricted command, in which
case "to make writing the ssh restricted command
script|implementation" or something, but that seems wordy.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>