Docs: fix wrong characters in npm init example with specific template

[lewandy]

Describe the bug

I just noticed that the example has wrong characters in flag template on https://vitejs.dev/guide/#scaffolding-your-first-vite-project .

Current

   npm init @vitejs/app my-vue-app -- --template vue

Suggestion

   npm init @vitejs/app my-vue-app --template vue

Reproduction

Go to https://vitejs.dev/guide/#scaffolding-your-first-vite-project and see the following.

[bwyx]

I notice that too yesterday its added by Evan himself so I thought it's done on purpose but I don't know
as in 3dd1187

[yyx990803]

Just double checked - the extra -- is required for npm 7+ but is not needed for npm 6.