MDL-58138 completion: Fix capability required for setting defaults

As per core_completion\manager::get_available_completion_tabs() the
required capability for this feature is moodle/course:manageactivities,
not moodle/course:update.

course/editdefaultcompletion.php

@@ -36,7 +36,7 @@
 $PAGE->set_heading($course->fullname);
 $PAGE->set_pagelayout('admin');
 
-require_capability('moodle/course:update', context_course::instance($course->id));
+require_capability('moodle/course:manageactivities', context_course::instance($course->id));
 
 // Prepare list of selected modules.
 $manager = new \core_completion\manager($course->id);