scp tutorial improvements

[Jeroen Ooms <jeroen@xxxxxxxxxxxx>]

The scp tutorial at http://api.libssh.org/master/libssh_tutor_scp.html
mentions SSH_SCP_REQUEST_NEWDIRECTORY and
SSH_SCP_REQUEST_ENDDIRECTORY. These should be SSH_SCP_REQUEST_NEWDIR
and SSH_SCP_REQUEST_ENDDIR instead.

Also the tutorial is a bit sparse, it would be helpful to link to
examples/scp_download.c on the git server. I found this example really
helpful when trying to understand how to scp directories.