Fix question reference in InstrumentToWarehouse spec.

The integrity checks added in f7679287f9378da7eab0efc608a349ad79b8 to
address #4084 make this necessary.  (That's a good thing.)

This is part of
https://code.bioinformatics.northwestern.edu/issues/issues/show/4379.
Implement MDES time and date coding.

Related to
https://code.bioinformatics.northwestern.edu/issues/issues/show/4379.
Merge pull request #231 from NUBIC/task-4379-mdes-datetime-coding-for-pbs

Export answers of -1, -2, etc. as MDES-coded dates / times (PBS)