Update 16-viz.mkd

File name is sprank.py not prank.py

Author: ghfudemytraining

book3/16-viz.mkd

@@ -263,7 +263,7 @@ For each iteration of the page rank algorithm it prints the average
 change in page rank per page. The network initially is quite unbalanced
 and so the individual page rank values change wildly between iterations.
 But in a few short iterations, the page rank converges. You should run
-*prank.py* long enough that the page rank values
+*sprank.py* long enough that the page rank values
 converge.
 
 If you want to visualize the current top pages in terms of page rank,