small doc update

doc/tutorial/extending_theano.txt

@@ -215,7 +215,7 @@ returns the right answer. If you detect an error, you must raise an
 
 We call ``utt.assert_allclose(expected_value, value)`` to compare
 NumPy ndarray.This raise an error message with more information. Also,
-we change change the tolerance with the Theano flags
+the default tolerance can be changed with the Theano flags
 ``config.tensor.cmp_sloppy`` that take values in 0, 1 and 2. The
 defaul value do the most strict comparison, 1 and 2 make less strict
 comparison.