Add AsyncMock to list of available modules from unittest.mock in documentation

[mishaschwartz]

When using unittest.mock as the mock_module, the AsyncMock class has been made accessible directly from the mocker fixture since v3.2.0:

pytest-mock/src/pytest_mock/plugin.py

Lines 50 to 51 in 9b6e106

	if hasattr(mock_module, "AsyncMock"):
	self.AsyncMock = mock_module.AsyncMock

I noticed that AsyncMock is not included in the list in the documentation so this PR adds it in.

[nicoddemus]

Thanks!