docs: fix grammatical error

[D-Sketon]

Check List

Please read and check followings before submitting a PR.

• [ ] I want to publish my theme on Hexo official website.
  • [ ] I have read the theme publishing doc.
  • [ ] name is unique.
  • [ ] link URL is correct.
  • [ ] preview URL is correct.
  • [ ] preview URL web site is rendered correctly.
  • [ ] Add a screenshot to source/themes/screenshots.
  • [ ] Screenshot filename is same as value of name.
  • [ ] Screenshot size is 800 * 500.
  • [ ] Screenshot file format is png.
• [ ] I want to publish my plugin on Hexo official website.
  • [ ] I have read the plugin publishing doc.
  • [ ] name is unique.
  • [ ] link URL is correct.
• [x] Others (Update, fix, translation, etc...)
  • Languages:
  • [x] en English
  • [ ] ru Russian
  • [ ] zh-cn simplified Chinese
  • [ ] zh-tw traditional Chinese

https://github.com/hexojs/site/pull/2077#discussion_r1348907453

[github-actions[bot]]

Theme name validation passed. Theme thumbnails validation completed.