remove/update alembic comments in migration template
The comment right now says:
# If you make any changes to the Exam table, please use
# a batch operation and supply the sqlite_autoincrement
# argument to preserve the AUTOINCREMENT keyword.
# See ./6b926be35894_exam_autoincrement.py for a reference.
The migration referenced was removed after updating to MySQL and probably, with this change, the batch operation is no longer need.