docs: fix multi-process QEMU documentation

Fix a typo in the system documentation for multi-process QEMU.

Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/system/multi-process.rst

@@ -4,7 +4,7 @@ Multi-process QEMU
 ==================
 
 This document describes how to configure and use multi-process qemu.
-For the design document refer to docs/devel/qemu-multiprocess.
+For the design document refer to docs/devel/multi-process.rst.
 
 1) Configuration
 ----------------