fixed typo in example gulp config

Author: Schweinepriester

README.md

@@ -186,7 +186,7 @@ module.exports = function(grunt) {
 var gulp = require('gulp');
 var rename = require('gulp-rename');
 var postcss = require('gulp-postcss');
-var selector = require('postcss-custom-selector')
+var selector = require('postcss-custom-selectors')
 var autoprefixer = require('autoprefixer-core')
 
 gulp.task('default', function () {