doc error in plugins.rst

in the example text:

In the ``CPUPollster`` plugin, the ``get_counters`` method is implemented as a loop
which, for each instances running on the local host, retrieves the cpu_time
from the hypervisor and sends back two ``Counter`` objects. The first one, named
"cpu", is of type "cumulative", meaning that between two polls, its value is
not reset, or in other word that the cpu value is always provided as a duration
that continuously increases since the creation of the instance. The second one,
named "instance", is of type "cumulative", meaning that it's value is just the
volume since the last poll. Here, the instance counter is only used as a way
to tell the system that the instance is still running, hence the hard coded
value of 1.

the second counter isn't named 'instance' nor is it cumulative.