Add priority to manual clean step example
Add priority to manual clean step example

clean_step decorator must have a priority argument, this change fixes
a docstring example of manual clean step that was missing it.

Change-Id: I6d6ca312fa3171026b2db427b648f7ae7273e6c2

SauloAislan/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,NaohiroTamura/ironic,dims/ironic,devananda/ironic,NaohiroTamura/ironic,pshchelo/ironic,openstack/ironic,bacaldwell/ironic,pshchelo/ironic,ionutbalutoiu/ironic,openstack/ironic,dims/ironic,SauloAislan/ironic