Fix item name font_size_formats

[MartinVanVan]

Description (*)

The item name that is defined in the adminhtml/di.xml for the Magento\PageBuilder\Model\Wysiwyg\DefaultConfigProvider is wrong. The solution is to change the item name from fontsize_formats to font_size_formats.
So the font size dropdown in the tinymce toolbar is showing the defined options.

Bug

• #886

Fixed Issues (if relevant)

1. Wrong config key fontsize_formats in di.xml results in missing font size option in page builder #886: Wrong config key fontsize_formats in di.xml results in missing font size option in page builder

Manual testing scenarios (*)

1. Apply patch ACSD-62755
2. create or edit a page/block
3. insert or select a text element
4. toolbar should show all defined font size options in dropdown

Checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)