Clarify how you would end up with None in programming.rst

Assigning `y.sort()` to a variable clarifies why you would end up with a `None`.

Author: alokmenghrajani

Doc/faq/programming.rst

@@ -481,7 +481,7 @@ object, whereas superficially similar operations (for example ``y = y + [10]``
 and :func:`sorted(y) <sorted>`) create a new object.  In general in Python (and in all cases
 in the standard library) a method that mutates an object will return ``None``
 to help avoid getting the two types of operations confused.  So if you
-mistakenly write ``y.sort()`` thinking it will give you a sorted copy of ``y``,
+mistakenly write ``z = y.sort()`` thinking it will give you a sorted copy of ``y``,
 you'll instead end up with ``None``, which will likely cause your program to
 generate an easily diagnosed error.