Fix links to docker hub in README.md

[thawn]

The links to docker hub still pointed to the deprecated images. This pull request updates the links to point to the current images.

Fixes issue #661

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[jameslamb]

Thanks very much for this! Could you also update the references in CONTRIBUTING.md?

I found those like this:

git grep -E 'rapidsai\-notebook'

[thawn]

@jameslamb thanks a lot for the code review!

I made the requested changes in CONTRIBUTING.md

[thawn]

Also searched myself again and did not find another occurrence of rapidsai-notebook

[thawn]

thanks for the tip with git grep -E did not know that one ;-)

[jameslamb]

Thanks for the help!

[raydouglass]

Thanks @thawn!

[raydouglass]

/merge