gnu: openmpi: Fix typo.

* gnu/packages/mpi.scm (openmpi)[inputs]: For UCX, check the supported
systems of UCX, not PSM2.

gnu/packages/mpi.scm

@@ -192,7 +192,7 @@ (define-public openmpi
              `(("psm2" ,psm2))
              '())
        ,@(if (and (not (%current-target-system))
-                  (member (%current-system) (package-supported-systems psm2)))
+                  (member (%current-system) (package-supported-systems ucx)))
              `(("ucx" ,ucx))
              '())
        ("rdma-core" ,rdma-core)