Supply correct return type for `sentryVitePlugin`

[Lutymane]

The factory function actually returns an array of plugins which caused issues with plugin management with an expectation it returns a single plugin. It should be noted either in docs, or properly typed to avoid issues for other users.

https://github.com/getsentry/sentry-javascript-bundler-plugins/blob/9c564544b72caaea48205ce77ba6b4ddf0af09d2/packages/vite-plugin/src/index.ts#L77

[chargome]

Hey @Lutymane, thanks for writing in – we'll have a look!