Fix validation error on cookbook name

Chef 11.4.0 and ruby 1.9.2p290

Fixes the following error when using this cookbook:
[2013-03-19T18:12:11+00:00] FATAL: Chef::Exceptions::ValidationFailed:
collectd_plugin[write_graphite] (collectd::client_graphite line 35) had
an error: Chef::Exceptions::ValidationFailed: Option cookbook must be a
kind of [String]!  You passed :collectd.