Update declarative_mixins.rst

Variables ta and tb should be instances of TypeA and TypeB respectively.

Author: serzey

doc/build/orm/declarative_mixins.rst

@@ -352,7 +352,7 @@ attribute of each ``StringAttribute`` instance.
 argument ``strings``, a list of strings::
 
     ta = TypeA(strings=['foo', 'bar'])
-    tb = TypeA(strings=['bat', 'bar'])
+    tb = TypeB(strings=['bat', 'bar'])
 
 This list will generate a collection
 of ``StringAttribute`` objects, which are persisted into a table that's