[FIX] make MEG section headings and ToC consistent to the EEG and iEEG specs#181

Merged

sappelhoff merged 2 commits intobids-standard:masterfrom robertoostenveld:ToC_meg

Apr 3, 2019

+17-16

Commits on Mar 18, 2019

made section headings and ToC consistent with eeg and ieeg specification
robertoostenveld
committed

Commits on Mar 27, 2019

shortened file extension as per https://github.com/bids-standard/bids-specification/pull/181/files#r266490960
robertoostenveld
committed