Fix comment on WithMessage function (#86)

errors.go

diff --git a/errors.go b/errors.go
index 960d7c7..584bb79 100644
--- a/errors.go
+++ b/errors.go
@@ -202,7 +202,7 @@
 }
 
 // WithMessage annotates err with a new message.
-// If err is nil, WithStack returns nil.
+// If err is nil, WithMessage returns nil.
 func WithMessage(err error, message string) error {
 	if err == nil {
 		return nil