Address nit in 79dac41fee

Address a nit in comment in 79dac41fee,
as it is the first patch of the series, fix it here together with other
possible nits in the future reviews to avoid rebases.

Part of blueprint abort-live-migration-in-queued-status

Change-Id: I2aa39fa38f2d6720b41289f7f710645f2cdb82be

nova/compute/manager.py

@@ -531,7 +531,7 @@ class ComputeManager(manager.Manager):
             # ncpu * 5 which is probably much higher than anyone would sanely
             # use for concurrently running live migrations.
             self._live_migration_executor = futures.ThreadPoolExecutor()
-        # This is a dict, keyed by migration uuid, to a two-item tuple of
+        # This is a dict, keyed by instance uuid, to a two-item tuple of
         # migration object and Future for the queued live migration.
         self._waiting_live_migrations = {}