docs: Fix devicetree command description
    
    Specifically fix the subsection and drop bogus reference to the GNU/Linux.
    
    Reported-by: Patrick Higgins <higgi1pt@gmail.com>
    Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
    Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>