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got rid of atomic_ops.rst, pointing that this was superseded by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.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/virt/kvm/vcpu-requests.rst

@@ -302,6 +302,6 @@ VCPU returns from the call.
 References
 ==========
 
-.. [atomic-ops] Documentation/core-api/atomic_ops.rst
+.. [atomic-ops] Documentation/atomic_bitops.txt and Documentation/atomic_t.txt
 .. [memory-barriers] Documentation/memory-barriers.txt
 .. [lwn-mb] https://lwn.net/Articles/573436/