Fix typo in doc comment for ExitStatusCode field

nagios.go

@@ -66,7 +66,7 @@ type ExitState struct {
  // check contents of Inbox").
  LastError error
 
- // LastError is the exit or exit status code provided to the Nagios
+ // ExitStatusCode is the exit or exit status code provided to the Nagios
  // instance that calls this service check. These status codes indicate to
  // Nagios "state" the service is considered to be in. The most common
  // states are OK (0), WARNING (1) and CRITICAL (2).