Add missing link to npm-package (#1494)

Adds a link to npm package flush-promises to link to additional documentation

Author: marina-mosti

docs/guides/testing-async-components.md

@@ -99,7 +99,7 @@ it('fetches async when a button is clicked', done => {
 
 The reason `setTimeout` allows the test to pass is because the microtask queue where promise callbacks are processed runs before the task queue, where `setTimeout` callbacks are processed. This means by the time the `setTimeout` callback runs, any promise callbacks on the microtask queue will have been executed. `$nextTick` on the other hand schedules a microtask, but since the microtask queue is processed first-in-first-out that also guarantees the promise callback has been executed by the time the assertion is made. See [here](https://jakearchibald.com/2015/tasks-microtasks-queues-and-schedules/) for a more detailed explanation.
 
-Another solution is to use an `async` function and the npm package `flush-promises`. `flush-promises` flushes all pending resolved promise handlers. You can `await` the call of `flushPromises` to flush pending promises and improve the readability of your test.
+Another solution is to use an `async` function and the [npm package flush-promises](https://www.npmjs.com/package/flush-promises). `flush-promises` flushes all pending resolved promise handlers. You can `await` the call of `flushPromises` to flush pending promises and improve the readability of your test.
 
 The updated test looks like this: