docs(README.md): update reference from vite-plugin-dts to unplugin-dts

[jpbnetley]

closes #1059

Checklist

• Title as described.
• Make sure you put things in the right category.
• The description of your item should be a sentence with less than 24 words.
• Avoid using links and parentheses in description.
• Omit unnecessary words already provided in the context (e.g. for Vite, a Vite plugin).
• Use proper case for terms (e.g. GitHub, TypeScript, ESLint, etc.)
• When mentioning tools, omit versions whenever possible (e.g. use TypeScript instead of TypeScript 4.x, but keep Vue 2 and Vue 3 as they're incompatible).
• When mentioning package names, use quotes whenever possible.
• Always add your items to the end of a list.

Plugins/Tools

• The plugin/tool is working with Vite 2.x and onward.
• The project is Open Source.
• The project follows the Vite Plugins Conventions.
• The plugin uses Vite-specific hooks and can't be implemented as a Compatible Rollup Plugin.
• The repo should be at least 30 days old.
• The documentation is in English.
• The project is active and maintained (inactive projects for longer 6 months will be removed without further notice).
• The project accepts contributions.
• Not a commercial product.

Starter Templates

• The starter template is working with Vite 2.x and onward.
• The documentation is in English.
• The starter template must provide enough instructions / documentation about how to start up and what's included.
• A screenshot or a live demo should be included.
• The repo should be at least 30 days old.
• Should be differentiable from the existing starter templates.

Apps/Websites

• The website is available without errors and load in a reasonable amount of time.
• The website must be Open Source and showing it's using Vite intensively.
• The website is original and not too simple. For that reason, blogs and simple landing pages are rejected.