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

pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go