Fixed genetic_algorithm() population iterator (aimacode#296)

Seems like a typo, results in error:
TypeError: 'int' object is not iterable.

Fixing it:
for i in len(population) -> for i in range(len(population))

Author: yrevar

search.py

@@ -584,7 +584,7 @@ def genetic_algorithm(population, fitness_fn, ngen=1000, pmut=0.1):
     "[Figure 4.8]"
     for i in range(ngen):
         new_population = []
-        for i in len(population):
+        for i in range(len(population)):
             fitnesses = map(fitness_fn, population)
             p1, p2 = weighted_sample_with_replacement(population, fitnesses, 2)
             child = p1.mate(p2)