Fix the sample name of 'resource_metadata'

on http://docs.openstack.org/developer/ceilometer/webapi/v2.html,
the sample of resource_metadata is empty because its name is wrong.

Change-Id: I18d8bfc7f3d7eecbc2c4071dadb379abbc966010
Fixes: bug #1183316
Merge "Fix the sample name of 'resource_metadata'"