Add missing word (GH-21936)

"data to lost" -> "data to be lost"

Doc/library/multiprocessing.rst

@@ -861,7 +861,7 @@ For an example of the usage of queues for interprocess communication see
 
       A better name for this method might be
       ``allow_exit_without_flush()``.  It is likely to cause enqueued
-      data to lost, and you almost certainly will not need to use it.
+      data to be lost, and you almost certainly will not need to use it.
       It is really only there if you need the current process to exit
       immediately without waiting to flush enqueued data to the
       underlying pipe, and you don't care about lost data.