Sosulski 2019 has an incorrect number of sessions

[guido151]

Unless I am mistaken, there is no mention of multiple sessions on the site of the Sosulski 2019 dataset (https://freidok.uni-freiburg.de/data/154576) nor in the article (https://arxiv.org/abs/2109.06011). However, in the code (https://github.com/NeuroTechX/moabb/blob/develop/moabb/datasets/sosulski2019.py#L17-L197) and on the site (http://moabb.neurotechx.com/docs/generated/moabb.datasets.Sosulski2019.html#moabb.datasets.Sosulski2019), the dataset is listed as having three sessions.

[PierreGtch]

As discussed during the MOABB meeting, indeed there is only one session so we should update the doc. However, we will keep the parameter use_soas_as_sessions to true by default to avoid unexpected results with the WithinSessionEvaluation.

@guido151 do you want to make a first contribution? :)