util: Fix cpt_upgrader format string

Change-Id: I9d15316f199b20976420c35d2c79dd13cc9db9ee
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44510
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

util/cpt_upgrader.py

@@ -126,7 +126,7 @@ class Upgrader:
                 sys.exit(1)
             Upgrader.untag_set.add(self.tag)
         else:
-            print("Error: no upgrader or downgrader method for".format(
+            print("Error: no upgrader or downgrader method for {}".format(
                 self.tag))
             sys.exit(1)