doc: do not slice array literal

doc: do not slice array literal

The special case in the spec is that you can take the
address of a composite literal using the & operator.

A composite literal is not, however, generally addressable,
and the slice operator requires an addressable argument,
so [3]int{1,2,3}[:] is invalid.  This tutorial code and one bug
report are the only places in the tree where it appears.

R=r, gri
CC=golang-dev
https://golang.org/cl/5437120