debug/dwarf: fix typos in comment for UnspecifiedType
debug/dwarf: fix typos in comment for UnspecifiedType

LGTM=iant, bradfitz
R=bradfitz, iant
CC=golang-codereviews
https://codereview.appspot.com/120700043

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