Need to adjust iEEG docs of MEF3 format

[sappelhoff]

In iEEG, the MEF3 format is currently described as if it were a single file with a .mef extension, but it's actually a directory with a .mefd "extension" and several other files therein.

The validator will be fixed in bids-standard/bids-validator#958, now we need to:

• finish validator PR cc @MaxvandenBoom (basically only needs one more review by Ross)
• adjust the specification text
• @dorahermes has offered to include an example in the BIDS starter kit, perhaps linking to the OpenNeuro dataset in MEF3 format that she currently prepares