Fixed mistake in test for f-string error description (GH-22036) (GH-22059)

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index b9bede0..b53661a 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -1217,7 +1217,7 @@
     def test_with_an_underscore_and_a_comma_in_format_specifier(self):
         error_msg = re.escape("Cannot specify both ',' and '_'.")
         with self.assertRaisesRegex(ValueError, error_msg):
-            f'{1:,_}'
+            f'{1:_,}'
 
 if __name__ == '__main__':
     unittest.main()