[Docs] Fix typo

[zhouzaida]

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Motivation

Fix typo #1111

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-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. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1166 (e7ae259) into master (b035fe9) will not change coverage.
The diff coverage is n/a.

❗ Current head e7ae259 differs from pull request most recent head 3295618. Consider uploading reports for the commit 3295618 to get more accurate results

@@           Coverage Diff           @@
##           master    #1166   +/-   ##
=======================================
  Coverage   68.01%   68.01%           
=======================================
  Files         159      159           
  Lines       10421    10421           
  Branches     1895     1895           
=======================================
  Hits         7088     7088           
  Misses       2966     2966           
  Partials      367      367           

Flag	Coverage Δ
unittests	68.01% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/utils/config.py	90.38% <ø> (ø)

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