Fix name of images at Python lesson

Related with #451.

@ethanwhite Could you review this?

~~~
$ git rev-parse HEAD
b212695cc0f228c48fd89a2fd5d716216112fbea
$ grep '\.png' novice/python/01-numpy.md 
<img src="../../novice/python/01-numpy_files/novice/python/01-numpy_71_0.png">
<img src="../../novice/python/01-numpy_files/novice/python/01-numpy_73_0.png">
<img src="../../novice/python/01-numpy_files/novice/python/01-numpy_75_1.png">
<img src="../../novice/python/01-numpy_files/novice/python/01-numpy_75_3.png">
<img src="../../novice/python/01-numpy_files/novice/python/01-numpy_80_0.png">
~~~