docs: Fix link to front-matter.md

[h3xx]

Bug repro:

• https://vuepress-theme-blog.billyyyyy3320.com/#summary
• Click on "Writing the summary manually in frontmatter"

Summary

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [x] Docs
• [ ] Build-related changes
• [ ] Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• [ ] Chrome
• [ ] Firefox
• [ ] Safari
• [ ] Edge
• [ ] IE

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature
• [ ] Related documents have been updated
• [ ] Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

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