top-k instead of top-p in MixtralConfig docstring

[sorgfresser]

What does this PR do?

The current docstring for the num_experts_per_tok is quite misleading - mixtral uses TopK to formulate their experts per token in their paper. While I think top-p would be interesting to see, the experts are currently routed using top-k. This PR addresses this.

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@ArthurZucker

[ArthurZucker]

thanks!

[HuggingFaceDocBuilderDev]

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