Model Selection typos and suggestions.

[anchal-physics]

Hi
Thanks for the wonderful presentation and this repo. There are minor typos in definition in Model Selection section in introduction.ipynb. I also feel it would be nicer to denote indices 2 and 1 both in subscript. I initially got confused thinking they are squared.
I also feel for sake of readers, some more explanation of how is calculated.

[ColmTalbot]

Thanks @anchal-physics, I'll aim to address these soon. I will say for now that the notation of B^2_1 is fairly standard (if confusing) so I'd rather keep it, but I'll add a comment to make it more clear.