fix: update path for lib/auth to match the rest of the code

Author: shrekuu

content/800-guides/230-betterauth-nextjs.mdx

@@ -351,7 +351,7 @@ touch "src/app/api/auth/[...all]/route.ts"
 Add the following code to the newly created `route.ts` file. This code uses a helper from Better Auth to create Next.js-compatible `GET` and `POST` request handlers.
 
 ```ts
-import { auth } from "@/lib/auth";
+import { auth } from "@/src/lib/auth";
 import { toNextJsHandler } from "better-auth/next-js";
 
 export const { POST, GET } = toNextJsHandler(auth);