Fix markdown version

[zytx121]

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Motivation

[Please describe the motivation of this PR and the goal you want to achieve through this PR.](https://readthedocs.org/projects/mmcv/builds/17455215/

Failed to build docs due to breaking changes of markdown3.4. More details at ryanfox/sphinx-markdown-tables#36.)https://readthedocs.org/projects/mmcv/builds/17455215/

Failed to build docs due to breaking changes of markdown3.4. More details at ryanfox/sphinx-markdown-tables#36.

Modification

Fix the markdown version.

BC-breaking (Optional)

Does the modification introduce changes that break the back-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. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #414 (e845ff2) into dev (36de5f6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #414   +/-   ##
=======================================
  Coverage   29.46%   29.46%           
=======================================
  Files         121      121           
  Lines        8495     8495           
  Branches     1289     1289           
=======================================
  Hits         2503     2503           
  Misses       5891     5891           
  Partials      101      101           

Flag	Coverage Δ
unittests	29.44% <ø> (ø)

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