docs: fix middleware path (#40340)

Hello,

Just update the docs to be aligned with the current middleware implementation 🙂

Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

docs/advanced-features/middleware.md

@@ -31,7 +31,7 @@ To begin using Middleware, follow the steps below:
 npm install next@latest
 ```
 
-2. Create a `middleware.ts` (or `.js`) file at the same level as your `pages` directory
+2. Create a `middleware.ts` (or `.js`) file at the root or in the `src` directory (same level as your `pages`)
 3. Export a middleware function from the `middleware.ts` file:
 
 ```typescript