testing: improve the usage message of -benchtime
Specify that -benchtime can take the form Nx.
Change-Id: I1e711cdb2e19e3ff5eb2cea4e7c8843bc58696b1
GitHub-Last-Rev:
1cb13f7dba829497c5cbb3d4c578f4984e1cbd28
GitHub-Pull-Request: golang/go#61756
Reviewed-on: https://go-review.googlesource.com/c/go/+/515801
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>