sweet: be explicit about how to get the repository

I always forget how to clone it to get started.

Change-Id: Ie3f04f8c99e73e89a56c70ef6a21750d43268b9b
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/670115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>