tour: improve sample code for methods with pointers
tour: improve sample code for methods with pointers

Fixes golang/go#12034

Change-Id: Ie63b389f7c8f7348112cb566fdc019784b2c653b
Reviewed-on: https://go-review.googlesource.com/13262
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

campoy/go-tour-ua,Go-zh/tour,LaPingvino/go-tour-nl,shuLhan/tour,shuLhan/tour,LaPingvino/go-tour-nl,phoorichet/tour,bbandaru/tour,LaPingvino/go-tour-nl,raindev/tour,phoorichet/tour,campoy/go-tour-ua,bbandaru/tour,campoy/go-tour-ua,zorion/go-tour-ca,shuLhan/go-tour-id,Go-zh/tour,golians/tour,tamnd/tour,shuLhan/tour,shuLhan/go-tour-id,LaPingvino/go-tour-eo,phoorichet/tour,dupoxy/go-tour-fr,bbandaru/tour,golang/tour,tamnd/tour,raindev/tour,Go-zh/tour,raindev/tour,golians/tour,LaPingvino/go-tour-eo,Go-zh/tour,LaPingvino/go-tour-eo,zorion/go-tour-ca,tamnd/tour,dupoxy/go-tour-fr,scryner/tour,golians/tour,scryner/tour,scryner/tour,scryner/tour,shuLhan/go-tour-id,dupoxy/go-tour-fr