standardized markdown for all headers

It seems like some headers are using `===`, some `=====` and some `========`.

Notice the issue that `ADVANCED: SYSTEM MODES` headings became the sub-set of EEprom above it.

http://docs.spark.io/firmware/#eeprom-automatic-mode
http://docs.spark.io/firmware/#eeprom-semi-automatic-mode
http://docs.spark.io/firmware/#eeprom-manual-mode

Not sure if this fixes the issue but most probably it does.