test/bench/garbage: update Benchmarks Game URL to new page

The existing URL in comment points to an Alioth page which was
deprecated (and not working), so use the new Benchmarks Game URL.

Change-Id: Ifd694382a44a24c44acbed3fe1b17bca6dab998f
Reviewed-on: https://go-review.googlesource.com/136835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/bench/garbage/tree.go

@@ -28,7 +28,7 @@ POSSIBILITY OF SUCH DAMAGE.
 */
 
 /* The Computer Language Benchmarks Game
- * http://shootout.alioth.debian.org/
+ * https://benchmarksgame-team.pages.debian.net/benchmarksgame/
  *
  * contributed by The Go Authors.
  * based on C program by Kevin Carson