"population change event" -> "population size change event"

docs/simulator.rst

@@ -23,7 +23,7 @@ initialize and run simulations:
   You can also specify which *simulator* to use.  By default there is one
   built-in simulator--a simple simulator named `.OneEventSimulator` which
   uses :ref:`msprime <msprime:sec_intro>` which generates a model with one
-  population change event.
+  population size change event.
 
   Additional simulator implementations can be provided via :ref:`plugins
   <extending>`.  Below we will give a brief :ref:`tutorial