Correct a minor typo in README.md
Do not store Nagios ssh check key in /var/tmp, may be periodically cleaned

Linux Filesystem Hierarchy Standard states "Although data stored in
/var/tmp is typically deleted in a site-specific manner, it is
recommended that deletions occur at a less frequent interval than
/tmp."  We do not want to recommend a location for the key which is
subject to possible cleanup by system administration tools.