MDL-57437 core: upgrade code for introduction of block_myoverview

Part of MDL-55611 epic.

lib/blocklib.php

@@ -2537,6 +2537,6 @@ function blocks_add_default_system_blocks() {
     }
 
     $newblocks = array('private_files', 'online_users', 'badges', 'calendar_month', 'calendar_upcoming');
-    $newcontent = array('lp', 'course_overview');
+    $newcontent = array('lp', 'myoverview');
     $page->blocks->add_blocks(array(BLOCK_POS_RIGHT => $newblocks, 'content' => $newcontent), 'my-index', $subpagepattern);
 }