doc/progs/slices.go: fix typo in comment
doc/progs/slices.go: fix typo in comment

Fixes issue 6025.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/12387046

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