docs: vcpu-requests.rst: fix reference for atomic ops

Changeset f0400a77 ("atomic: Delete obsolete documentation")
got rid of atomic_ops.rst, pointing that this was superseded by
Documentation/atomic_*.txt.

Update its reference accordingly.

Fixes: f0400a77 ("atomic: Delete obsolete documentation")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/703af756ac26a06c2185c05dfe6d902253f11161.1621413933.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>