Fix filename typo

[philparzer]

What does this PR do?

Fixes typo in filename of .md file; changed "docs/mutli_gpu.md" to "docs/multi_gpu.md" and updated link in README.md accordingly

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

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