docs(cpus): correct default of CPUs usage

[mstruebing]

Background

In the Readme is stated that the number of CPUs used defaults to 8 which isn't correct, see:

vegeta/main.go

Line 24 in 0cabfc9

cpus := fs.Int("cpus", runtime.NumCPU(), "Number of CPUs to use")

BTW: is the commit message okay or how should I name it?

Checklist

• Git commit messages conform to community standards.
• Each Git commit represents meaningful milestones or atomic units of work.
• Changed or added code is covered by appropriate tests.

[tsenart]

This is just the output of running the command in my machine.

[mstruebing]

But on another machine it would be a different number, so I've updated the readme correctly.

[mstruebing]

Can someone restart travis?