cmd/gc: use appropriate verb to print array type length.
cmd/gc: use appropriate verb to print array type length.

Fixes issue 4730.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8229043

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