Update PHPDoc @Covers tag in theme tests

Replaced @coversDefaultClass with @Covers for better specificity in theme invalidateHeaderImagesCache test. This adjustment enhances the precision of test coverage annotation and maintains the accuracy of test documentation. No actual code or functionality was altered.

tests/phpunit/tests/theme/invalidateHeaderImagesCache.php

@@ -2,7 +2,7 @@
 /**
  * @group themes
  *
- * @coversDefaultClass ::invalidate_header_images_cache
+ * @covers ::invalidate_header_images_cache
  */
 class Tests_Theme_invalidateHeaderImagesCache extends WP_UnitTestCase {
 	public $custom_image_header;