README.rst: Fix a bug in Python code

`map()` returned a list in Python 2 but returns a generator in Python 3.
```python
>>> planes = ('', 'a', 'ab', 'abc')
>>> map(len, planes)
<map object at 0x1031e74c0>
>>> [len(plane) for plane in planes]
[0, 1, 2, 3]
```

Author: cclauss

README.rst

@@ -88,5 +88,5 @@ Converting images:
     >>> img2.get_pixel_format()
     'yuv420p'
     >>> planes = img2.to_bytearray()
-    >>> map(len, planes)
+    >>> [len(plane) for plane in planes]
     [50000, 12500, 12500, 0]