net: fix docs for type IP: s/array/slice/.
net: fix docs for type IP: s/array/slice/.

R=rsc
CC=golang-dev
https://codereview.appspot.com/6930047

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