Documentation: atomic_ops.txt is core-api/atomic_ops.rst

I was reading the memory barries documentation in order to make sure the
RISC-V barries were correct, and I found a broken link to the atomic
operations documentation.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/memory-barriers.txt

@@ -498,11 +498,11 @@ And a couple of implicit varieties:
      This means that ACQUIRE acts as a minimal "acquire" operation and
      RELEASE acts as a minimal "release" operation.
 
-A subset of the atomic operations described in atomic_ops.txt have ACQUIRE
-and RELEASE variants in addition to fully-ordered and relaxed (no barrier
-semantics) definitions.  For compound atomics performing both a load and a
-store, ACQUIRE semantics apply only to the load and RELEASE semantics apply
-only to the store portion of the operation.
+A subset of the atomic operations described in core-api/atomic_ops.rst have
+ACQUIRE and RELEASE variants in addition to fully-ordered and relaxed (no
+barrier semantics) definitions.  For compound atomics performing both a load
+and a store, ACQUIRE semantics apply only to the load and RELEASE semantics
+apply only to the store portion of the operation.
 
 Memory barriers are only required where there's a possibility of interaction
 between two CPUs or between a CPU and a device.  If it can be guaranteed that