adjust message displayed on Done To-Dos tab to reflect API v3 changes

The 30 most recently completed To-Dos are now shown, regardless of how long ago they were completed.

common/locales/en/tasks.json

@@ -1,6 +1,6 @@
 {
     "clearCompleted": "Delete Completed",
-    "lotOfToDos": "Completed To-Dos are automatically archived after 3 days. You can access them from Settings > Export.",
+    "lotOfToDos": "Your most recent 30 completed To-Dos are shown here. You can see older completed To-Dos from Data > Data Display Tool or Data > Export Data > User Data.",
     "deleteToDosExplanation": "If you click the button below, all of your completed To-Dos and archived To-Dos will be permanently deleted. Export them first if you want to keep a record of them.",
     "addmultiple": "Add Multiple",
     "addsingle": "Add Single",