Added comma to syntax error in bind.pp

Ran into a minor problem while trying to use the bind plugin.

Error: Could not parse for environment production: Syntax error at 'memorystats'; expected ')' at /home/dlawrence/code/puppet/modules/collectd/manifests/plugin/bind.pp:5
Error: Try 'puppet help parser validate' for usage

This patch fixes the issue by adding a comma to the ensure line.