[Docs]Change the code about installation

[qingtian5]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDetection or MMClassification.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[PeterH0323]

Hi @suibe-qingtian
I leave some suggestion for the doc, you can click the commit suggestion button if you accept the suggestion change.
Thx for your contribute. 😄

[PeterH0323]

Hi @suibe-qingtian

Pls modifiy the English version at the same time:

• https://github.com/open-mmlab/mmyolo/blob/main/README.md

And I just find out another wrong code in get_started.md, pls do the same change for it :

• https://github.com/open-mmlab/mmyolo/blob/main/docs/en/get_started.md
• https://github.com/open-mmlab/mmyolo/blob/main/docs/zh_cn/get_started.md

[PeterH0323]

LGTM

[PeterH0323]

Hi @suibe-qingtian

Pls change merge branch from main to dev

[PeterH0323]

LGTM

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