godoc: don't display navigation list with only 1 element
godoc: don't display navigation list with only 1 element

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/4547056

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