Parallel run example in README produces an error

[javierbrea]

The example in the README.md file about parallel runs contains an error. The fs.writeFileSync requires at least two arguments, and in the example only one is passed. So, it would not work if copied as it is.

It is desired to add a second argument in the example, an empty string would be enough to make it work.

This issue was originally discovered by @marboleda in #244