Use integer division to avoid errors in indexing

Fixes dipy#1215

doc/examples/brain_extraction_dwi.py

@@ -61,7 +61,7 @@
 import matplotlib.pyplot as plt
 from dipy.core.histeq import histeq
 
-sli = data.shape[2] / 2
+sli = data.shape[2] // 2
 plt.figure('Brain segmentation')
 plt.subplot(1, 2, 1).set_axis_off()
 plt.imshow(histeq(data[:, :, sli].astype('float')).T,