ch3: applied.Rmd answer needs to be edited

[umbersar]

File: https://github.com/asadoughi/stat-learning/blob/master/ch3/applied.Rmd
Answer: 15c.

The text of the answer says "Coefficient for nox is approximately -10 in univariate model and 31 in
multiple regression model.".

But it should be the other way around: "Coefficient for nox is approximately 31 in univariate model and -10 in multiple regression model."