doc/go_spec: fix broken link

        Part of issue 3424

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5955049

doc/go_spec.html

@@ -1598,7 +1598,7 @@ Given a set of identifiers, an identifier is called <i>unique</i> if it is
 <i>different</i> from every other in the set.
 Two identifiers are different if they are spelled differently, or if they
 appear in different <a href="#Packages">packages</a> and are not
-<a href="Exported_identifiers">exported</a>. Otherwise, they are the same.
+<a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
 </p>
 
 <h3 id="Constant_declarations">Constant declarations</h3>