go-tour: add discussion of pointers and some small tweaks

- Mention Sprint instead of Print when discussing recursive stringers.
- Remove discussion of the "new" function; it's rarely used today.
- Better hint in the cube root exercise (Who knows the cube root of 2?).
- Use v to refer to Vertex and p to refer to a pointer.

LGTM=campoy
R=campoy
CC=golang-codereviews
https://golang.org/cl/117840043