[release-branch.r60] doc: update tutorial.

««« CL 5175052 / e144a6dec55e
doc: update tutorial.
Fix for new regexp library ($ isn't end of line any more).
Don't assume . is in PATH.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5175052
»»»

R=dsymonds
CC=golang-dev
https://golang.org/cl/5199042