Typos and grammar changes to Chapters 2-4

[jhrcook]

This is a very interesting book! As I was reading, I noticed a few typos in the text and LaTeX (such as using ~ instead of \sim) and some places where I would suggest minor grammatical changes. Let me know if you would like anything changed back, but hopefully you find this PR helpful.

Thanks again for providing this book as a great resource on SEM, Josh

P.S. If possible, can you add the hacktoberfest-accepted label to this PR?