doc/articles/slices_usage_and_internals: fix typo         Fixes issue 3753.
doc/articles/slices_usage_and_internals: fix typo
        Fixes issue 3753.

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/6304097

wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old