correct small error in models code

Former-commit-id: 86cbfb7ecd83e8560695d670c9ed6cf9c67ebfe4

ippisite/ippidb/models.py

@@ -434,7 +434,7 @@ class CompoundAction(models.Model):
         get the complexes involved in the compound action
         which are always the bound complexes
         """
-        return ppi.get_ppi_bound_complexes()
+        return self.ppi.get_ppi_bound_complexes()
 
 class RefCompoundBiblio(models.Model):
     compound = models.ForeignKey(Compound)