Fix broken links

Exercises/ex3_2.md

@@ -66,7 +66,7 @@ price 490.1
 
 ## (c) Table Output
 
-In link:ex3_1.html[Exercise 3.1], you wrote a function `print_portfolio()`
+In [Exercise 3.1](ex3_1.md), you wrote a function `print_portfolio()`
 that made a nicely formatted table.   That function was custom tailored
 to a list of `Stock` objects.  However, it can be completely generalized
 to work with any list of objects using the technique in part (b).