Decrease send_instructor_email_digest_tasks() batch size

Decrease the `batch_size` of the `send_instructor_email_digest_tasks()`
Celery task.

This is because I want to try decreasing the task's `batch_size` and
correspondingly increasing its `rate_limit` (see
hypothesis/lms#5827) to see what effect that has
on performance, with an eye towards potentially removing the batching
feature altogether in order to simplify adding future features to this
code.

h_periodic/lms_beat.py

@@ -26,7 +26,7 @@
         "send_instructor_email_digests": {
             "task": "lms.tasks.email_digests.send_instructor_email_digest_tasks",
             "schedule": crontab(hour=7, minute=15),
-            "kwargs": {"batch_size": 25},
+            "kwargs": {"batch_size": 10},
         },
         "delete_expired_task_done_rows": {
             "task": "lms.tasks.task_done.delete_expired_rows",