Fix typo in function set_maintenance

Fix a simple typo in the `set_maintenance` function in the `Setting` class.

The function does not seem to be actually used anywhere.

app/models.py

@@ -1731,7 +1731,7 @@ class Setting(db.Model):
         self.name = name
         self.value = value
 
-    def set_mainteance(self, mode):
+    def set_maintenance(self, mode):
         maintenance = Setting.query.filter(Setting.name=='maintenance').first()
 
         if maintenance is None: