openstack: Fix image name in README
-# ![](./img/integrations_openstack.png) OpenStack
+# ![](./img/integration_openstack.png) OpenStack
Metadata associated with the OpenStack plugin for collectd can be found [here](https://github.com/signalfx/integrations/tree/release/collectd-openstack). The relevant code for the plugin can be found [here](https://github.com/signalfx/collectd-openstack).
