[Docs] Update mmcls doc

[Renzhihan]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily receiving feedbacks. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

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Modification

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[CLAassistant]

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[Renzhihan]

I am not sure what to fill in for PPLNN in the readme. I have kept it consistent with before. Please give me some advice on how to improve it.

[RunningLeon]

If you have time, you could install pplnn and check if the models could be deployed on pplnn successfully. If not, could keep them as original.
Besides, you could update https://github.com/open-mmlab/mmdeploy/blob/main/docs/zh_cn/04-supported-codebases/mmcls.md

[RunningLeon]

LGTM