tour: add note regarding := with map two value assignment
tour: add note regarding := with map two value assignment

Change-Id: I5c6b820e7dc0afa2d007f93c72fdb603c1d65d65
Reviewed-on: https://go-review.googlesource.com/12714
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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