Update es.py (#627)

pymoo/algorithms/soo/nonconvex/es.py

@@ -45,7 +45,7 @@ def __init__(self,
         if pop_size is None and n_offsprings is not None:
             pop_size = int(np.math.ceil(n_offsprings * rule))
         elif n_offsprings is None and pop_size is not None:
-            n_offsprings = int(np.math.floor(n_offsprings / rule))
+            n_offsprings = int(np.math.floor(pop_size / rule))
 
         assert pop_size is not None and n_offsprings is not None, "You have to at least provivde pop_size of n_offsprings."
         assert n_offsprings >= 2 * pop_size, "The number of offsprings should be at least double the population size."