Added default value doc string

Author: amilbourne

pandas/_testing.py

@@ -949,7 +949,7 @@ def assert_numpy_array_equal(
     obj : str, default 'numpy array'
         Specify object name being compared, internally used to show appropriate
         assertion message.
-    index : numpy.ndarray
+    index : numpy.ndarray, default None
         optional index (shared by both left and right), used in output.
     """
     __tracebackhide__ = True