pluginapi: Drop mention of Naemon 3

I'm guessing this is just an accidental sed gone wild or something.
Anyways, I don't feel we're obliged to mention historical changes to
the Nagios API either way, so let's just say that multiple output is
optional and be done with it.

Let's keep it simple, eh?

Signed-off-by: Anton Lofgren <alofgren@op5.com>