debug/dwarf: fix typos in comment for UnspecifiedType

LGTM=iant, bradfitz
R=bradfitz, iant
CC=golang-codereviews
https://golang.org/cl/120700043

src/pkg/debug/dwarf/type.go

@@ -88,7 +88,7 @@ type AddrType struct {
 	BasicType
 }
 
-// A UnspecifiedType represents implicit, unknown, ambiguous or nonexistent type.
+// An UnspecifiedType represents an implicit, unknown, ambiguous or nonexistent type.
 type UnspecifiedType struct {
 	BasicType
 }