doc/codewalk/markov: fix slice error in description Fixes issue 5176.
doc/codewalk/markov: fix slice error in description
Fixes issue 5176.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8118046

firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old