Fix to Chapter 3 solution: stats.py, us_population_stats_figure_2.png

chapter3/solutions/us_population_stats_figure_2.png is not correct
because the population growth list is unintentionally sorted by
`median()` in stats.py.

This commit replaces `numbers.sort()` in `median()` with
`sorted(numbers)`, which does not modify the list itself but returns a
new sorted list, and fixes us_population_stats_figure_2.png.

Before this commit:
    >>> data = [3, 2, 1]
    >>> median(data)
    >>> print(data) #=> [1, 2, 3] (data is sorted.)

After this commit:
    >>> data = [3, 2, 1]
    >>> median(data)
    >>> print(data) #=> [3, 2, 1] (data is not sorted.)

Author: vzvu3k6k

chapter3/solutions/stats.py

@@ -18,7 +18,7 @@ def median(numbers):
     N = len(numbers)
 
     # sort the list in ascending order
-    numbers.sort()
+    numbers = sorted(numbers)
 
     # find the median
     if N % 2 == 0: