Deadlink in dev/gpu.md

[rjkilpatrick]

https://github.com/fastai/fastai-docs/blob/5ed291976a23d15144e713cd347ecd3d1c295fd8/dev/gpu.md#L205

Links to https://docs.fast.ai/tutorial.resources.html which does not exist. I believe this should link to https://docs.fast.ai/learner, but learn.purge is not documented there, which after a look through the source, I assume fastai2 has no learn.purge method and this is a remnant from fastai1