README.rst: Fix a bug in Python code

[cclauss]

map() returned a list in Python 2 but returns an iterator in Python 3.

• https://docs.python.org/3/whatsnew/3.0.html#views-and-iterators-instead-of-lists

map() and filter() return iterators. If you really need a list and the input sequences are all of equal length, a quick fix is to wrap map() in list(), e.g. list(map(...)), but a better fix is often to use a list comprehension (especially when the original code uses lambda), or rewriting the code so it doesn’t need a list at all.

>>> planes = ('', 'a', 'ab', 'abc')
>>> map(len, planes)
<map object at 0x1031e74c0>
>>> [len(plane) for plane in planes]
[0, 1, 2, 3]