config: Fix an error message in Port.splice().

That error message referenced non-existent variables which were likely
renamed without updating the error message.

Change-Id: I6878802ef4b83e3fdf75a860d848b8c5e2e8d6c0
Reviewed-on: https://gem5-review.googlesource.com/c/15515
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/python/m5/params.py

@@ -1805,7 +1805,7 @@ class PortRef(object):
             else:
                 raise TypeError, \
                       "Splicing non-port references '%s','%s' to port '%s'"\
-                      % (new_peer, peers_new_peer, self)
+                      % (new_master_peer, new_slave_peer, self)
         else:
             fatal("Port %s not connected, cannot splice in new peers\n", self)