[i18n] gen.php: mar to mrt (#1623)

See https://github.com/FreshRSS/FreshRSS/pull/1617#discussion_r134110655

app/i18n/nl/gen.php

@@ -77,8 +77,8 @@ return array(
 		'last_month' => 'Vorige maand',
 		'last_week' => 'Vorige week',
 		'last_year' => 'Vorig jaar',
-		'mar' => 'mar',
-		'march' => 'Mar',
+		'mar' => 'mrt',
+		'march' => 'Mrt',
 		'may_' => 'Mei',
 		'may' => 'Mei',
 		'mon' => 'Ma',