doc: fix effective_go: s/byte array/byte slice/.
doc: fix effective_go: s/byte array/byte slice/.

R=rsc
CC=golang-dev, mdempsky
https://codereview.appspot.com/7062049

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang