[x/tour] README.md: add badge to pkg.go.dev
[x/tour] README.md: add badge to pkg.go.dev

Change-Id: Ifa508b83e5b02db6a3d5a173879fab17bdd4a50d
Reviewed-on: https://go-review.googlesource.com/c/tour/+/275196
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
X-Tour-Commit: 004403599411fcd726b1e58cd0199083507d2047

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website