Corrections to running_ssh_service.md

File: [engine/examples/running_ssh_service.md](https://docs.docker.com/engine/examples/running_ssh_service/)

The following RUN line should be changes:
    from
RUN sed -i 's/PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config
    to
RUN sed -i 's/PermitRootLogin [a-zA-Z-]*/PermitRootLogin yes/' /etc/ssh/sshd_config

thus making it agnostic of image setting...
