[Docs] Fix mmseg document

[AI-Tianlong]

Fix mmseg document
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After：

[CLAassistant]

All committers have signed the CLA.

[RunningLeon]

@AI-Tianlong hi, thanks for your PR.

1. Could sign the CLA
2. Could fix lint error by running pip install pre-commit and pre-commit run --all-files and commit new changes.

[AI-Tianlong]

@AI-Tianlong hi, thanks for your PR.

1. Could sign the CLA
2. Could fix lint error by running pip install pre-commit and pre-commit run --all-files and commit new changes.

I'll do it right away

[RunningLeon]

LGTM