spec: better comment in example for type definition

The old comment for the example

	type PtrMutex *Mutex

talked about the method set of the base type of PtrMutex.
It's more direct and clearer to talk about the underlying
type of PtrMutex for this specific example.
Also removed link inside pre-formatted region of text.

Fixes #20900.

Change-Id: Ie37340e53670e34ebe13e780ba8ccb1bba67795c
Reviewed-on: https://go-review.googlesource.com/55070Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of June 28, 2017",
+	"Subtitle": "Version of August 14, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -1946,7 +1946,7 @@ func (m *Mutex) Unlock()  { /* Unlock implementation */ }
 // NewMutex has the same composition as Mutex but its method set is empty.
 type NewMutex Mutex
 
-// The method set of the <a href="#Pointer_types">base type</a> of PtrMutex remains unchanged,
+// The method set of PtrMutex's underlying type *Mutex remains unchanged,
 // but the method set of PtrMutex is empty.
 type PtrMutex *Mutex