corrected spelling of Notendurchschnittsrechner in app modules and grade averages view

lib/state/app/modules/app_modules.dart

@@ -107,7 +107,7 @@ class AppModule {
       ),
       Modules.gradeAveragesCalculator: AppModule(
         Modules.gradeAveragesCalculator,
-        name: 'Notendurschnittsrechner',
+        name: 'Notendurchschnittsrechner',
         icon: () => Icon(Icons.calculate),
         breakerArea: BreakerArea.more,
         create: GradeAveragesView.new,