Docs incorrectly describe `createNotifier()` as a middleware

[markerikson]

The docs say that ReactPerformance.createNotifier() returns a "middleware", but it actually creates a Redux store enhancer instead, which is why it needs to be passed to compose() instead of passed to applyMiddleware(). This should be fixed for clarity.

[amsul]

@markerikson true! I'd love a PR 🙂