Fix Documentation User-Defined Mutation Operator Example: the number of genes is the offspring shape dimension index 1.

Author: augustodamasceno

docs/source/README_pygad_ReadTheDocs.rst

@@ -2261,7 +2261,7 @@ gene's value.
    def mutation_func(offspring, ga_instance):
 
        for chromosome_idx in range(offspring.shape[0]):
-           random_gene_idx = numpy.random.choice(range(offspring.shape[0]))
+           random_gene_idx = numpy.random.choice(range(offspring.shape[1]))
 
            offspring[chromosome_idx, random_gene_idx] += numpy.random.random()