Typos in the lectures

• Today: at least at the end, the line redux, I wrote p¡¤x where I intended p¡¤v. It doesn't matter to the substance, but I failed to be consistent in the notation, given that I had "x" and "y" on the coordinate axes.
• Likely bug two weeks ago, second-to-last-ish line of the deduction solving the x' = rx - k differential equation; check if you are missing an exponential function.

- Nils