doc: fix codewalk/markov.go
doc: fix codewalk/markov.go

When I build it, I got:
/home/njubee/work/golang/doc/codewalk/markov.go:124: undefined: time.Nanoseconds
time.Nanoseconds() does not exist now, use time.Now().UnixNano() instead

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/5668044

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

gnanderson/go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme