crosperf: fix --help

This is a format string; without this, --help raises
ValueError: unsupported format character ',' (0x2c) at index 311

BUG=None
TEST=crosperf --help

Change-Id: I5fbdb996888156f8a1531c2b140c3f734be7edff
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2173547
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
1 file changed