spec: refer to "not defined type" rather than "unnamed type" in conversions

We missed this in https://golang.org/cl/36213.
Thanks to Chris Hines for pointing it out.

For #18130.

Change-Id: I6279ab19966c4391c4b4458b21fd2527d3f949dd
Reviewed-on: https://go-review.googlesource.com/36691Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of February 7, 2017",
+	"Subtitle": "Version of February 9, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -3908,7 +3908,8 @@ in any of these cases:
 	</li>
 	<li>
 	ignoring struct tags (see below),
-	<code>x</code>'s type and <code>T</code> are unnamed pointer types
+	<code>x</code>'s type and <code>T</code> are pointer types
+	that are not <a href="#Type_definitions">defined types</a>,
 	and their pointer base types have identical underlying types.
 	</li>
 	<li>