Fix minor docstring formats

[zhouzaida]

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Motivation

Fix typo

Modification

Fix docstring

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 #1285 (98a5993) into master (ac0d839) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1285   +/-   ##
=======================================
  Coverage   68.42%   68.42%           
=======================================
  Files         160      160           
  Lines       10610    10610           
  Branches     1938     1938           
=======================================
  Hits         7260     7260           
  Misses       2975     2975           
  Partials      375      375           

Flag	Coverage Δ
unittests	68.42% <ø> (ø)

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

Impacted Files	Coverage Δ
mmcv/image/photometric.py	99.27% <ø> (ø)

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