apply fix for exercise reference (#136)

Author: mmcky

lectures/python_by_example.md

@@ -475,7 +475,7 @@ Set $T=200$ and $\alpha = 0.9$.
 
 ### Exercise 2
 
-Starting with your solution to exercise 2, plot three simulated time series,
+Starting with your solution to exercise 1, plot three simulated time series,
 one for each of the cases $\alpha=0$, $\alpha=0.8$ and $\alpha=0.98$.
 
 Use a `for` loop to step through the $\alpha$ values.