Updated comment on metric parameter

Fixed comment where example metric was:

[ "%{@source_host}/uptime", %{uptime_1m} " ]

This syntax will cause an error everytime. Updated to:

[ "%{@source_host}/uptime", "%{uptime_1m}" ]
Merge pull request #457 from eravenelle/patch-1

Updated comment on metric parameter for graphite output
Merge git://github.com/logstash/logstash