Update docs

[WenjieDu]

What does this PR do?

Updating docs

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).

[coveralls]

Pull Request Test Coverage Report for Build 13479918343

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 82.241%

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Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/csdi/model.py	1	85.07%

Totals
Change from base Build 13479906524:	-0.7%
Covered Lines:	11716
Relevant Lines:	14246

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💛 - Coveralls