clarify the example in 'Simulation with Actions' RST document

In RST document congress/doc/source/enforcement.rst.

The policy 'aliceactions' is created at first in the example, but that is never used, all the following command use the default 'action' policy, that make the users confused about which policy we should create "action('set')" rule into and which action policy we should use in simulate? Aims to clarify the usage, avoid to use default 'action' policy, and use 'aliceactions' policy in the example.

2.1 Simulation with Actions
    $ openstack congress policy create aliceactions --kind 'action'
    $ openstack congress policy rule create action 'action("set")'
    $ openstack congress policy rule create action 'p+(x,y) :- set(x,y)'
    $ openstack congress policy rule create action 'p-(x,oldy) :- set(x,y), p(x,oldy)'

a) **Inserts and Deletes**. Set key 101 to value 5 and ask for the contents of error::

    $ openstack congress policy simulate classification 'error(x)' 'set(101, 5)' action
    error(302)