[Fix] Update the cn doc

[lianqing01]

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

Motivation

update the getting_started in cn doc.

[CLAassistant]

All committers have signed the CLA.

[yaqi0510]

Dear lianqing01,

First of all, we want to express our gratitude for your significant PR in the MMDetection3d project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

If you are Chinese or have WeChat，welcome to join our community on WeChat. You can add our assistant ：openmmlabwx. Please add "mmsig + Github ID" as a remark when adding friends：）

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Thank you again for your contribution❤

Best regards！@lianqing01