[x/tour] gotour: rename Vertex variables to v
[x/tour] gotour: rename Vertex variables to v

Fixes #12.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5308049
X-Tour-Commit: 25eb531f7da326fc74ecec237b610ff80ca9bd61

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website