Callers of md5.Sum should do so to avoid allocations, the example did not demonstate this property.

««« original CL description
crypto/md5: add example for Sum

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/64820044

»»»

LGTM=minux.ma
R=r, minux.ma
CC=golang-codereviews
https://golang.org/cl/65180043