Fix spelling (#19145)

Fixes up #19138

changelog.d/19145.misc

@@ -0,0 +1 @@
+Minor speed up of processing of inbound replication.

synapse/replication/tcp/handler.py

@@ -70,7 +70,7 @@ from synapse.replication.tcp.streams._base import (
     DeviceListsStream,
     ThreadSubscriptionsStream,
 )
-from synapse.util.bacckground_queue import BackgroundQueue
+from synapse.util.background_queue import BackgroundQueue
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer

tests/util/test_background_queue.py

@@ -19,7 +19,7 @@ from twisted.internet.defer import Deferred
 from twisted.internet.testing import MemoryReactor
 
 from synapse.server import HomeServer
-from synapse.util.bacckground_queue import BackgroundQueue
+from synapse.util.background_queue import BackgroundQueue
 from synapse.util.clock import Clock
 
 from tests.unittest import HomeserverTestCase