minor typos in chapter 8

[rpruim]

• Here are some questions to ask to gain diagnostic insights we gain from Figure 8.2. --> The enumerated list consists of three statements an no questions.
• estimating the causal effect within propensity score stratum --> strata or perhaps "within each propensity score stratum"
• Notice here the matchit function fit a logistic regression model --> fits
• we would have predicted that they were exposed and but by chance they were not, or vice-versa --> but