Edits for Memory Chapter

Hey @arturo, here are my edits for your Memory chapter. Again, most of my edits are inline/**[BD: noted]**. The only other thing that I will say is that I did not change variableNames to `variableNames` in the text, but I think that this is advised. I can go back over the chapter and do that for you if you'd like, but I thought I should mention it first. Also, I made no attempt to standardize the we/you language but I suggest that one is picked. Other than that the chapter is quite solid. I think it is the most finished (after the edits) of the ones of yours that I have read. Great work!