Update examples/MNIST

[ablaom]

Using latest pkg releases, the example will possibly fail for GPUs because of scalar indexing implicit in the make2d function (the use of a colon argument in reshape). Either use

function make2d(x)
    l = length(x)
    b = size(x)[end]
    reshape(x, div(l, b), b)
end

Or MLUtils.flatten which does the same thing.