docs: fix config filename for Production build section (#7926)

Co-authored-by: Vladimir <[email protected]>

Author: gwennlbh

docs/guide/in-source.md

@@ -55,8 +55,10 @@ $ npx vitest
 
 For the production build, you will need to set the `define` options in your config file, letting the bundler do the dead code elimination. For example, in Vite
 
-```ts [vitest.config.ts]
-import { defineConfig } from 'vitest/config'
+```ts [vite.config.ts]
+/// <reference types="vitest/config" />
+
+import { defineConfig } from 'vite'
 
 export default defineConfig({
   test: {