[Docs] Fix the version of sphinx

[zhouzaida]

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Motivation

As recommended by readthedocs, we should fix the version of sphinx.

Modification

Fix the version of sphinx

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
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

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #1301 (e47f07d) into master (7540cf7) will not change coverage.
The diff coverage is n/a.

❗ Current head e47f07d differs from pull request most recent head 053a574. Consider uploading reports for the commit 053a574 to get more accurate results

@@           Coverage Diff           @@
##           master    #1301   +/-   ##
=======================================
  Coverage   68.27%   68.27%           
=======================================
  Files         161      161           
  Lines       10742    10742           
  Branches     1972     1972           
=======================================
  Hits         7334     7334           
  Misses       3023     3023           
  Partials      385      385           

Flag	Coverage Δ
unittests	68.27% <ø> (ø)

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