doc/comedilib.ent: Correct 'comedi_date'
doc/comedilib.ent: Correct 'comedi_date'

Use the date that doc/drivers.txt was actually last generated
(2021-05-13) instead of today's date (2021-07-28).

Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib,Linux-Comedi/comedilib