[release-branch.go1.11] doc: update docs.html with new tour import path

As of golang.org/cl/141857 the import path has changed from
golang.org/x/tour/gotour to golang.org/x/tour

Change-Id: Ib54ab2e50188ef66c8a5c45136babfa49ad6934a
Reviewed-on: https://go-review.googlesource.com/c/141917


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 035f9e81)
Reviewed-on: https://go-review.googlesource.com/c/143617