fix: wrong danish language code to da (#44)

Author: vincentdoerig

lang/dk.css lang/da.csslang/dk.css renamed to lang/da.css

@@ -41,7 +41,7 @@ <h5>Abstract</h5>
       <!-- ↓ Sort alphabetically ↓ -->
       <option value="cs">Czech (cs)</option>
       <option value="de">German (de)</option>
-      <option value="dk">Danish (dk)</option>
+      <option value="da">Danish (da)</option>
       <option value="es">Spanish (es)</option>
       <option value="fr">French (fr)</option>
       <option value="hu">Hungarian (hu)</option>