spec: consistently use "indices" (rather than "indexes")

spec: consistently use "indices" (rather than "indexes")

We have been using all three terms "indices", "indexes",
and "index expressions" indiscriminatly for index values.
With this change, "index" refers to an index value,
"indices" is the plural of "index", and "index expression"
refers to an array, slice, or map indexed by an index: a[x].

R=r, rsc, iant, ken, mtj
CC=golang-dev
https://golang.org/cl/6912056