docs: return promise in async test example

[cdoublev]

Summary

The callback of the test example for expect.assertions() is failing because it does not return a promise.

Alternatively, the description below the test could say:

The expect.assertions(2) call ensures that both callbacks actually get called. It will fail in the example above, because the promise is not returned therefore Jest will not wait for doAsync() to run the callbacks.

Test plan

[linux-foundation-easycla]

• ✅login: cdoublev / (92499b3)

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