encoding/json: clarify correct usage of struct tags in associated article.

Fixes #4297.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/6817045

doc/articles/json_and_go.html

@@ -130,7 +130,7 @@ preference):
 
 <ul>
 <li>
-An exported field with a tag of <code>"Foo"</code> (see the
+An exported field with a tag of <code>`json:"Foo"`</code> (see the
 <a href="/ref/spec#Struct_types">Go spec</a> for more on struct tags),
 </li>
 <li>