Fix self.fail() call in test_data_header

Lib/test/test_urllibnet.py

@@ -184,7 +184,7 @@ class urlretrieveNetworkTests(unittest.TestCase):
             try:
                 time.strptime(datevalue, dateformat)
             except ValueError:
-                self.fail('Date value not in %r format', dateformat)
+                self.fail('Date value not in %r format' % dateformat)
 
     def test_reporthook(self):
         records = []