fix CIFAR100 docs class number error (#149)

Author: cluffa

src/datasets/vision/cifar100.jl

@@ -47,8 +47,8 @@ end
 
 The CIFAR100 dataset is a labeled subsets of the 80
 million tiny images dataset. It consists of 60000
-32x32 colour images in 10 classes, with 6000 images
-per class.
+32x32 colour images in 100 classes and 20 superclasses,
+with 600 images per class.
 
 Return the CIFAR-100 **trainset** labels (coarse and fine)
 corresponding to the given `indices` as a tuple of two `Int` or