[libc++][docs] Use --show-all in the sample command-line to run benchmarks

It's really not useful at all to run benchmarks without --show-all since
you don't get the benchmark output. And since --show-all is the suggested
default way to run benchmarks, it's not necessary anymore to mention it
right below.

GitOrigin-RevId: e84f79e925bf49600e24e0d11e20c282037e4a2c