MDL-55548 core: fixed misleading comment in upgrade code

lib/db/upgrade.php

@@ -2869,7 +2869,7 @@ function xmldb_main_upgrade($oldversion) {
         $table = new xmldb_table('course_sections');
         $field = new xmldb_field('timemodified', XMLDB_TYPE_INTEGER, '10', null, XMLDB_NOTNULL, null, '0', 'availability');
 
-        // Define a field 'timemodified' in the 'course_sections' table, to background deletion tasks.
+        // Define a field 'timemodified' in the 'course_sections' table.
         if (!$dbman->field_exists($table, $field)) {
             $dbman->add_field($table, $field);
         }