playground: mention vet in about text
playground: mention vet in about text

Fixes golang/go#25462

Change-Id: I11b861352b53f9492cd1f5c6d8c434edd68e4bd9
Reviewed-on: https://go-review.googlesource.com/c/140497
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/playground,golang/playground,golang/playground