doc: change example with respect to other convert options

[atennert]

Input and output are usually defined at the end of the convert command. If '-' for stdin is not listed after the options, then for instance '-background none' will not work.

[atennert]

I was wondering whether to also include an example for file type conversion or replace the current one. It would look like this:

// example using ImageMagick's convert
// setting "buffer: false" optional but recommended for heavy I/O
gulp.src('./src/images/*.{jpg,gif,svg}', {buffer: false})
    .pipe(spawn({
      cmd: 'convert',
      args: [
        '-background', 'none',
        '-resize', '50%',
        '-',
        'png:-'
      ],
      // optional
      opts: {cwd: '.'},
      filename: (base, ext) => `${base}-half.png`
    }))
    .pipe(gulp.dest('./dist/images'));

It looks more complicated than the current one though ...