Update collectd.conf.pod

spotted by lukas227 in IRC:
23:43 < lukas227> Hi, I think I found a mistake in the threshold.conf(5) man page: the example
                  THRESHOLD CONFIGURATION is within <Threshold></Threshold> tags, but should
                  probably be within <Plugin "threshold"></Plugin> tags (doesn work with
                  <Threshold> for me)
Day changed to 27 Sep 2013

Signed-off-by: Florian Forster <octo@collectd.org>