Fix typo in test_wp_loading_optimization_force_header_contexts_filter

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: westonruter

tests/phpunit/tests/media.php

@@ -4921,7 +4921,7 @@ public function test_wp_loading_optimization_force_header_contexts_filter() {
 
 		add_filter(
 			'wp_loading_optimization_force_header_contexts',
-			function ( $context ) {
+			function ( $contexts ) {
 				$contexts['something_completely_arbitrary'] = true;
 				return $contexts;
 			}