Remove 'JOBS=0' from disable parallelization section of README

[charlespierce]

The README incorrectly states that setting JOBS=0 will disable parallelization. The check in parallel-api.js for the JOBS environment variable is:

const JOBS = Number(process.env.JOBS) || require('os').cpus().length;

Number(0) is a falsey value, so setting JOBS=0 is equivalent to not setting JOBS at all, and will default to the number of CPUs. Setting JOBS=1 does correctly disable parallelization, so that should be the only suggested approach.

[Turbo87]

Good catch.thanks!