edit the wrong code in doc 'delete-files-folder.md'

Created by: osdio

In the older doc, the way to delete file in pipeline is wrong .

So I change to the right way.

The right way to delete pipeline should like this:

var gulp = require('gulp');
var stripDebug = require('gulp-strip-debug'); // only as an example
var del = require('del');
var vinylPaths = require('vinyl-paths');

gulp.task('clean', function () {
    var vp=vinylPaths();

     gulp.src('1.js')
     .pipe(vp)
    .pipe(stripDebug())
    .pipe(gulp.dest('dist'))
    .on('end',function(){
        del(vp.paths);
    })
});

gulp.task('default', ['clean']);