Added missing self-parameter to change_credit

cash/models.py

@@ -27,7 +27,7 @@ class Account(models.Model):
 				instance.account.card_number = instance.ldap_user.attrs['employeenumber'][0]
 				instance.account.save()
 
-	def change_credit(amount, subject, desc):
+	def change_credit(self, amount, subject, desc):
 		self.credit -= amount
 		self.save()