benchmark: improve compare.R output

Before:

events/ee-emit.jslisteners=1 argc=0 n=10                -2.31 %       ±3.60% ±4.79% ±6.24%

After:

events/ee-emit.js listeners=1 argc=0 n=10                -2.31 %       ±3.60% ±4.79% ±6.24%

FWIW I'm not an R expert, but this seemed like the correct solution.