Fixed a failute caused by transaction-package changes.

Weird, I thought I'd tested the transaction changes against ZODB. :/

doc/guide/transactions-and-threading.rst

@@ -187,7 +187,7 @@ So, for example, if we wanted to set a transaction note::
 .. -> src
 
    >>> exec(src)
-   >>> db.history(conn.root()._p_oid)[0]['description']
+   >>> str(db.history(conn.root()._p_oid)[0]['description'])
    'incrementing x again'
 
 Here, we used the