Outdated DataLoader tutorial

The [Dataloader documentation] points to a tutorial page, https://fluxml.ai/tutorials/2021/01/21/data-loader.html,
which is a bit outdated. In particular, the old MLDatasets.jl interface is used
```julia
x, y = MNIST.traindata(Float32)
dataloader = DataLoader((x, y))
```
instead of the new one
```julia
x, y = MNIST()[:]
dataloader = DataLoader((x, y))
```