[Observium] Minor documentation typo

Hi team,

There is a minor typo on this page:

http://www.observium.org/docs/unix_agent/

Specifically this,

scp /opt/observium/scripts/observium_agent_xinetd user@your_server:/etc/xinet.d/observium_agent_xinetd

Note that /etc/xinet.d should be /etc/xinetd.d

Very minor, but thought I'd let you know about it.