Minor typo suggestion

On the rendered website, the console output for a code chunk (lines 439-445) should be

#> a + b + x

But is instead

#> 1 + 2 + mpg

I am not sure how to fix this, but wanted to point it out. Appears to be a quirk in how knitr converts .Rmd to .md, but I do not know enough about knitr internals to know for sure.
Manually fixed output for R code chunk

Due to quirks in R's substitute() combined with how knitr converts .Rmd
to .md, the R output was "incorrect". So I changed {r} to {r, eval =
FALSE} and manually inserted the desired output as a comment.

Also: my prior commit incorrectly noted that lines 439-445 needed
changes. Should instead be lines 431-434.
Merge pull request #193 from liangcj/patch-1

Minor typo suggestion