fixed instructions for legacy plugin name

[yairkukielka]

this PR fixes the instructions for the plugin name.

the current instructions

plugins: ['vitest']

give this error when running npm run lint in a package:

Oops! Something went wrong! :(

ESLint: 9.8.0

ESLint couldn't find the plugin "eslint-plugin-vitest".

(The package "eslint-plugin-vitest" was not found when loaded as a Node module from the directory "/Users/yair/p/vite-app-template".)

It's likely that the plugin isn't installed correctly. Try reinstalling by running the following:

    npm install eslint-plugin-vitest@latest --save-dev
...

[veritem]

LGTM!!!