Fix incorrect documented usage

[s100]

The documented example usage of this package is backwards. Using the example code as it is currently, here's what happens:

mkdir i-should-be-empty
touch i-should-be-empty/Thumbs.db
touch i-should-be-empty/.DS_Store

const emptyDir = require('empty-dir');

function filter(filepath) {
  return !/(Thumbs\.db|\.DS_Store)$/i.test(filepath);
}

console.log(emptyDir.sync('./i-should-be-empty', filter));

This prints:

false

when clearly this directory should be considered "empty" due to containing nothing but unwanted garbage files.

Compare with the filter used during unit testing, which is correct:

empty-dir/test/index.js

Lines 13 to 15 in 4817e65

	function isGarbageFile(filename) {
	return /(?:Thumbs\.db|\.DS_Store)$/i.test(filename);
	}

[phated]

Makes sense to me! Thanks!