updates Functions.rmd

* harmonizes some capitalization
* numbers (v. bullets) for Exercises
* fixes some links
* puts in some output carets, ">" for consistency

On this last point on the carets (lines 705, 709) it is not clear to me why you made this chunk with `eval = FALSE`, but I figure you have your reasons. Maybe so the file doesn't nuisance-change when compiling?

* removes `eval = FALSE` from the first part of "Return values" because you are not asking us to imagine the output.

### Other questions:

1. line 325: Can you expand on the difference between using single quotes and double quotes? My previous understanding is that they are interchangeable. Are they for all other practical purposes? Is there a recommended set of practices?

2. line 817: May not be the place to discuss, but is it possible to improve R's implementation of copy-on-modify so as to reduce the performance hit? Does this provide some of the motivation for dplyr?

3. around line 868: It seems that `<-` is an exception to the default left-to-right parsing of infix operators. Is this worth pointing out?