Fix Chinese Documentation and Sphinx Version Bug (#718)

* fix version of sphinx

* fix switch

docs/switch_language.md

+## <a href='https://mmdetection3d.readthedocs.io/en/latest/'>English</a>
+
+## <a href='https://mmdetection3d.readthedocs.io/zh_CN/latest/'>简体中文</a>

docs_zh-CN/switch_language.md

+## <a href='https://mmdetection3d.readthedocs.io/en/latest/'>English</a>
+
+## <a href='https://mmdetection3d.readthedocs.io/zh_CN/latest/'>简体中文</a>

requirements/docs.txt

 m2r
 opencv-python
 recommonmark
-sphinx==3.1.2
+sphinx
 sphinx_markdown_tables
 sphinx_rtd_theme