doc: note go tool tour removal in 1.12 release notes
doc: note go tool tour removal in 1.12 release notes

Note the removal of the go tool tour command in the Go 1.12 release
notes.

Updates #24819

Change-Id: I258ab9401ea2cc06a83328c67299376fcf23c980
Reviewed-on: https://go-review.googlesource.com/c/158618
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,golang/go,akutz/go,josharian/go,josharian/go,josharian/go,akutz/go,golang/go,nawawi/go,CAFxX/go,golang/go,akutz/go,akutz/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,akutz/go,josharian/go,nawawi/go,akutz/go,nawawi/go,cloudflare/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,josharian/go,golang/go,nawawi/go,CAFxX/go