Fix outdated free variables syntax

[hugoabonizio]

Currently the example returns this error

$ crystal classification_example.cr 
Error in classification_example.cr:10: instantiating 'Array(Array(Float32))#shape()'

puts "Shapes: X: #{x.shape}, y: #{y.shape}"
                     ^~~~~

in /home/hugo/gh/crystal-learn/lib/array.cr:130: undefined constant T

def get_shape(arr : Array(Array(T)))
                                ^