go-tour: Stringer should be an interface, not a struct

Fixes #184.

LGTM=minux
R=campoy, minux
CC=golang-codereviews
https://golang.org/cl/183100043
diff --git a/content/methods.article b/content/methods.article
index 59d988b..a5881bb 100644
--- a/content/methods.article
+++ b/content/methods.article
@@ -65,7 +65,7 @@
 
 One of the most ubiquitous interfaces is [[//golang.org/pkg/fmt/#Stringer][`Stringer`]] defined by the [[//golang.org/pkg/fmt/][`fmt`]] package.
 
-	type Stringer struct {
+	type Stringer interface {
 		String() string
 	}