fix: Fixes missing PHP opening tag

Adds the PHP opening tag to prevent potential parsing errors
and ensure correct script execution.

lang/en/report_usergrades.php

@@ -1,3 +1,5 @@
+<?php
+
 $string['pluginname'] = 'User Grades Report';
 $string['usergradesreport'] = 'User Grades Report';
 $string['nouserselected'] = 'No user selected. Please select a user to view their grades.';