docs: Add await to response.json() for async ApiResponse class example

[Ekwinder]

The current async APIResponse class documentation in API Reference has an incorrect line in the example -

assert response.json()["name"] == "foobar"

As the async APIResponse response.json() returns a coroutine, this line will raise a TypeError: 'coroutine' object is not subscriptable exception.

Replacing it with

json_data = await response.json()
assert json_data["name"] == "foobar"

will solve this and show the correct way to use response.json()