spec: Fix broken type identity link

Fixes #7003.

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/47780043

doc/go_spec.html

@@ -1009,7 +1009,7 @@ A field declaration may be followed by an optional string literal <i>tag</i>,
 which becomes an attribute for all the fields in the corresponding
 field declaration. The tags are made
 visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
-and take part in <a href="Type_identity">type identity</a> for structs
+and take part in <a href="#Type_identity">type identity</a> for structs
 but are otherwise ignored.
 </p>