dc-download: use short date string

In both places in the UI where we show the date of a dive during
download we are actually pressed for space. So let's use the short
version of the date string to save some space.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

core/qthelper.cpp

@@ -1009,7 +1009,7 @@ QString get_short_dive_date_string(timestamp_t when)
 
 char *get_dive_date_c_string(timestamp_t when)
 {
-	QString text = get_dive_date_string(when);
+	QString text = get_short_dive_date_string(when);
 	return copy_qstring(text);
 }