lang: fix spelling mistake in translations of "feedback"

lang/main-de.json

             "chat": "Chatfenster ein-/ausblenden",
             "document": "Geteiltes Dokument schliessen",
             "download": "Unsere Apps herunterladen",
-            "feedback": "Feedback hinterlasen",
+            "feedback": "Feedback hinterlassen",
             "fullScreen": "Vollbildschirm ein-/ausblenden",
             "hangup": "Anruf beenden",
             "help": "Hilfe",
         "enterTileView": "Kachelansicht einschalten",
         "exitFullScreen": "Vollbildmodus verlassen",
         "exitTileView": "Kachelansicht ausschalten",
-        "feedback": "Feedback hinterlasen",
+        "feedback": "Feedback hinterlassen",
         "hangup": "Verlassen",
         "help": "Hilfe",
         "invite": "Teilnehmer einladen",