[FIX] Clarify that EDF/BDF files MUST have lower case extensions in B…

…IDS (#927)

* Fix EDF spec

* drop latin, use 'lower case' without dash -

* Update src/04-modality-specific-files/03-electroencephalography.md

Co-authored-by: Chris Markiewicz <effigies@gmail.com>

* Convert to table

* Format table

* add missing dots

Co-authored-by: Richard Höchenberger <richard.hoechenberger@gmail.com>

* add missing code formatting, one consistency fix

* Fix reviews

* STY: Fix typo, table fences

Co-authored-by: Stefan Appelhoff <stefan.appelhoff@mailbox.org>
Co-authored-by: Chris Markiewicz <effigies@gmail.com>
Co-authored-by: Richard Höchenberger <richard.hoechenberger@gmail.com>
Co-authored-by: Christopher J. Markiewicz <markiewicz@stanford.edu>