Do not call classes with full path it will result in not found error

Problem/Motivation

The markdown filter doesn't work if CommonMark is selected as the parser.

This problem is caused by the following part:

Markdown.php:

namespace Drupal\markdown\Plugin\Filter;

...

        case 'commonmark':
          $converter = new League\CommonMark\CommonMarkConverter();
          $text = $converter->convertToHtml($text);
          break;

The steps to reproduce:

1. Enable the module.
2. Add composer package league/commonmark to the project.
3. Add an text filter with markdown filter with CommonMark parser.
4. Try to save a new node with the text filter.
5. You can see the exception occurs.

Proposed resolution

Change the line

          $converter = new League\CommonMark\CommonMarkConverter();

to

          $converter = new CommonMarkConverter();