Fix redirect to correct date after editing substitution.

Closes #28.

biscuit/apps/chronos/views.py

@@ -153,7 +153,7 @@ def edit_substitution(request: HttpRequest, id_: int, week: int) -> HttpResponse
             edit_substitution_form.save(commit=True)
 
             messages.success(request, _('The substitution has been saved.'))
-            return redirect('lessons_day_by_date', when=week_days(week)[lesson_period.period.weekday].strftime('%Y-%m-%d'))
+            return redirect('lessons_day_by_date', when=week_days(week)[lesson_period.period.weekday - 1].strftime('%Y-%m-%d'))
 
     context['edit_substitution_form'] = edit_substitution_form