ruby: MI protocol: add a missing transition

The transition for state MII and event Store was found missing during testing.
The transition is being added. The controller will not stall the Store request
in state MII

src/mem/protocol/MI_example-cache.sm

@@ -413,7 +413,7 @@ machine(L1Cache, "MI Example L1 Cache")
 
   // TRANSITIONS
 
-  transition({IS, IM, MI, II}, {Load, Ifetch, Store, Replacement}) {
+  transition({IS, IM, MI, II, MII}, {Load, Ifetch, Store, Replacement}) {
     z_stall;
   }