Update Dutch (nl) strings (#4480)

app/javascript/mastodon/locales/nl.json

@@ -94,8 +94,8 @@
   "home.column_settings.show_replies": "Reacties tonen",
   "home.settings": "Kolom-instellingen",
   "lightbox.close": "Sluiten",
-  "lightbox.next": "Next",
+  "lightbox.next": "Volgende",
-  "lightbox.previous": "Previous",
+  "lightbox.previous": "Vorige",
   "loading_indicator.label": "Laden…",
   "media_gallery.toggle_visible": "Media wel/niet tonen",
   "missing_indicator.label": "Niet gevonden",