[1.8.x] Fixed a typo in the managers docs.

Backport of e03798a4 from master

docs/topics/db/managers.txt

@@ -309,11 +309,11 @@ returns a *subclass* of your base ``Manager`` with a copy of the custom
             return
 
     class MyModel(models.Model):
-        objects = BaseManager.from_queryset(CustomQueryset)()
+        objects = BaseManager.from_queryset(CustomQuerySet)()
 
 You may also store the generated class into a variable::
 
-    CustomManager = BaseManager.from_queryset(CustomQueryset)
+    CustomManager = BaseManager.from_queryset(CustomQuerySet)
 
     class MyModel(models.Model):
         objects = CustomManager()