ctdb-doc: Make example NFS Ganesha call-out almost pass shellcheck

ctdb-doc: Make example NFS Ganesha call-out almost pass shellcheck

This call-out is referred to in the wiki as an example, so make it as
clean as possible.

All the changes here are trivial.  There is still one failure, which
is slightly non-trivial, so it can be fixed in a separate commit.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>