Fixed a typo on page 72. A_k was a_k.

Hey pennatil
I'm still trying to get the hang of git and github but I think this should be alright.
My student email adresse is buschjo@student.ethz.ch.
About the change, it's pretty straightforward, the a_k was small and should be big. The way it was didn't make sense since a_k is an Element and A_k and A_{k+1} are sets. Also, to me, the notes look like both A_{k+1} and A_k are the same height.
Please also don't hesistate to make suggestions on how I should go about with pull requests in the future. Is it okay to make changes in the master branch or should I use a topic branch?
Have a nice day
Joel Busch