benchmark: remove redundant +

PR-URL: https://github.com/nodejs/node/pull/17803
Refs: https://github.com/nodejs/code-and-learn/issues/72


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>