tiny change, fix incorrect reference to SSH config variable "ClientAliveInterval"

Confusingly, the SSH client's keepalive parameter is "ServerAliveInterval" (ie, how often to check the server is alive), whereas "ClientAliveInterval" is something you would set server-side.

https://unix.stackexchange.com/questions/3026/what-do-the-options-serveraliveinterval-and-clientaliveinterval-in-sshd-conf

http://unixhelp.ed.ac.uk/CGI/man-cgi?ssh_config+5

http://unixhelp.ed.ac.uk/CGI/man-cgi?sshd_config+5