benchmark: use "confidence" in output of compare.R

Use the word "confidence" to indicate the confidence level of
the p value so it's easier to understand.
With this change more stars in the output of compare.R means
higher confidence level (lower significance level).

PR-URL: https://github.com/nodejs/node/pull/10737
Refs: https://github.com/nodejs/node/pull/10439


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>