bug: maxLevel calculated incorrectly

[Ore4444]

Summary

Which package does this PR involve? (check one)

• [ ] root project
• [ ] vuepress-plugin-clean-urls
• [ ] vuepress-plugin-container
• [ ] vuepress-plugin-dehydrate
• [ ] vuepress-plugin-medium-zoom
• [ ] vuepress-plugin-named-chunks
• [ ] vuepress-plugin-nprogress
• [ ] vuepress-plugin-redirect
• [ ] vuepress-plugin-smooth-scroll
• [x] vuepress-plugin-table-of-contents
• [ ] vuepress-plugin-typescript
• [ ] vuepress-plugin-zooming
• [ ] vuepress-mergeable
• [ ] vuepress-types

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

• [x] Bugfix (fix)
• [ ] Feature (feat)
• [ ] Performance enhancement (perf)
• [ ] Code Style (style)
• [ ] Refactor (refactor)
• [ ] Docs (docs)
• [ ] Build-related changes (build)
• [ ] CI-related changes (ci)
• [ ] Testing (test)
• [ ] Other, please describe:

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)

Other information:

[meteorlxy]

Uh, thanks. But it's a "range" rather than a "list".

E.g.

[2, 3] means "from 2 to 3", but not "2 and 3".

Maybe we should specify it in the docs.

[meteorlxy]

The related docs has been updated in 20bf7ea812194a23f64cdd371fbb747b8ae60e6a