doc: Adjust reference to spice-gtk man page for remote-viewer
    
    Similar to last commit, as noticed by reporter in bugzilla:
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1477966
    
    Man page should reference spice-client, not spice-gtk.
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>
    Acked-by: Victor Toso <victortoso@redhat.com>