Update langDE.ts

The current mothAbbr does not follow ISO8601, which is "Mrz" instead of "Mar" in germany.

Please see also datejs implementation: https://github.com/datejs/Datejs/blob/master/build/date-de-DE.js#L9

Another option would be to use "März" instead of "Mrz" - but "Mar" ist not correct.

src/i18n/langDE.ts

@@ -28,7 +28,7 @@ export default {
             'Juli', 'August', 'September', 'Oktober', 'November', 'Dezember'
         ],
         monthAbbr: [
-            'Jan', 'Feb', 'Mar', 'Apr', 'Mai', 'Jun',
+            'Jan', 'Feb', 'Mrz', 'Apr', 'Mai', 'Jun',
             'Jul', 'Aug', 'Sep', 'Okt', 'Nov', 'Dez'
         ],
         dayOfWeek: [