Issue #3460246 by Prem Suthar, joachim, afeijo: incorrect docs in DateFormatter::format()

(cherry picked from commit 5f17c3a2)

core/lib/Drupal/Core/Datetime/DateFormatter.php

@@ -127,7 +127,7 @@ public function format($timestamp, $type = 'medium', $format = '', $timezone = N
       }
     }
 
-    // Fall back to the 'medium' date format type if the format string is
+    // Fall back to the 'fallback' date format type if the format string is
     // empty, either from not finding a requested date format or being given an
     // empty custom format string.
     if (empty($format)) {