Update readme.md with CssMinFilter

[fred460]

Hello,
The last version CssMinFilter with wikimedia, the class is:
`<?php

namespace Assetic\Filter;

use Assetic\Contracts\Asset\AssetInterface;
use Wikimedia\Minify\CSSMin;

/**

• Filters assets through WikiMedia\Minify's CSSMin CSS minifier.
• @see https://github.com/wikimedia/minify/blob/master/src/CSSMin.php
  */
  class CSSMinFilter extends BaseFilter
  {
  public function filterDump(AssetInterface $asset)
  {
  $asset->setContent(CSSMin::minify($asset->getContent()));
  }
  }`

And the example in readme use "CssMinFilter" but is "CSSMinFilter".

[LukeTowers]

@fred460 can you submit a PR with your proposed change? I don't really understand what you're saying here.