fix($plugin-medium-zoom): disable zoom for links

[kidonng]

Summary

Disable zoom for links (e.g. "This project is sponsored by bit" on https://v1.vuepress.vuejs.org/guide/).

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

• Bugfix
• Feature
• Code style update
• Refactor
• 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)

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• No

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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)

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

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Other information:

[vue-bot]

Hey @kidonng, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚