i18n: Fix date format explanations for translators

The comments explaining date formatting to translators have a couple of typos related to the day name variables (%A and %a).

This commit fixes them.

Closes #1377

src/gui/gcal-event-widget.c

@@ -439,7 +439,7 @@ gcal_event_widget_update_timestamp (GcalEventWidget *self)
 
       if (gcal_event_get_all_day (self->event) || gcal_event_is_multiday (self->event))
         /*
-         * Translators: %A is the abbreviated day name, %B is the month name
+         * Translators: %a is the abbreviated day name, %B is the month name
          * and %d is the day of the month as a number between 0 and 31.
          * More formats can be found on the doc:
          * https://docs.gtk.org/glib/method.DateTime.format.html

src/gui/views/gcal-agenda-view.c

@@ -201,7 +201,7 @@ new_date_header_string (GDateTime *date)
     return g_strdup (_("Yesterday"));
   else
     /*
-     * Translators: %A is the abbreviated day name, %B is the month name
+     * Translators: %A is the full day name, %B is the month name
      * and %d is the day of the month as a number between 0 and 31.
      * More formats can be found on the doc:
      * https://docs.gtk.org/glib/method.DateTime.format.html