IB/rxe: Update documentation link

All Soft-RoCE (rxe) is handled now in rdma-core user space library,
so the documentation. The patch below updates the documentation
link to that new location.
Reported-by: Josh Beavers <josh.beavers@gmail.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/sw/rxe/Kconfig

@@ -22,4 +22,4 @@ config RDMA_RXE
 	To configure and work with soft-RoCE driver please use the
 	following wiki page under "configure Soft-RoCE (RXE)" section:
 
-	https://github.com/SoftRoCE/rxe-dev/wiki/rxe-dev:-Home
+	https://github.com/linux-rdma/rdma-core/blob/master/Documentation/rxe.md