fix mistake in db downgrade notes for 0.17

We need to add back the lab_controller.user_id index in the same ALTER
TABLE statement, because the column is involved in an InnoDB foreign key
and InnoDB requires an index on it.

    ERROR 1553 (HY000): Cannot drop index 'uc_user_id': needed in a foreign key constraint

Change-Id: Idff5fce194fca70c58a6dbf22cf048635895e8e2