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>