Change on Chapter 6 Question 7

[andylikescodes]

Hi there, I made a few changes on chapter 6 question 7:

1. I added a few lines in (a) to show how to get the distribution of Y. I also changed the math notation of the likelihood function to make it more consistent with the posterior in the book.
2. I changed Y_i to y_i, X_ij to x_ij to make it more consistent with the math notations in the book.

I only made changes to the .md file as I do not know how to convert it to .html and .Rmd properly.

If you find my changes useful, please merge.

Thanks.