cmd/bench: replace benchmarks-50 with benchmarks-100

it's the same number of benchmarks, the name is just
more accurate now.

Change-Id: Icf80a1ceec4de935788d77e1307cb849f00cca2d
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/808580
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>