fix bad link to distributed-training how-to guide in optimum-neuron docs

[aws-amj]

What does this PR do?

The "how-to" guides overview page has a bad link to the distributed training guide.

Before submitting

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

[HuggingFaceDocBuilderDev]

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