Fixed a typo in the Knitr Process Figure (3.4 in this version)

Hi Christopher!
Lovely Book.  And I really like the fact that all your sources are up on GitHub.  

Reading through it, there seemed to be a typo in figure 3.4.  I gave it a tweak.  Pull it in if my correction looks right.

Thanks again for all your hard work on this book.

Best,

eric