docs(plugin-mathjax): update macros code demo

[franklinqin0]

The old code mathjax: { gives me a lint error, thus I mimicked the code in "Usage" to fix error.

Summary

Which package does this PR involve? (check one)

• [ x ] 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
• [ ] 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:

[franklinqin0]

Actually on a second thought, could have the example similar to packages/docs/src/.vuepress/config.js#L52

The originally code lacks a quote anyway

[franklinqin0]

Also, should fix the Chinese doc as well: packages/docs/src/zh/plugins/mathjax.md#L149

[meteorlxy]

Thanks for your contribution ❤️

[lock[bot]]

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