docs: fix typo

[ghost]

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[pouchrobot]

Thanks for your contribution. 🍻 @moyasu
While we thought PR TITLE could be more specific, longer than 20 chars.
Please edit this PR title instead of opening a new one.
More details, please refer to https://github.com/dragonflyoss/Dragonfly/blob/master/CONTRIBUTING.md

[pouchrobot]

We found this is your first time to contribute to Dragonfly, @moyasu
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/dragonflyoss/Dragonfly/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[codecov-io]

Codecov Report

Merging #1287 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1287      +/-   ##
==========================================
+ Coverage   49.73%   49.74%   +0.01%     
==========================================
  Files         120      120              
  Lines        7938     7938              
==========================================
+ Hits         3948     3949       +1     
  Misses       3666     3666              
+ Partials      324      323       -1     

Impacted Files	Coverage Δ
supernode/daemon/mgr/scheduler/manager.go	22.60% <0.00%> (+0.68%)	⬆️

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[allencloud]

LGTM