Fix broken links

Exercises/ex6_4.md

@@ -60,7 +60,7 @@ There are no weird hacks involving a special `_init()` method or stack frames.
 
 ## (b) Creating an `__init__()` function
 
-In link:ex6_3.txt[Exercise 6.3], you wrote code that inspected the
+In [Exercise 6.3](ex6_3.md), you wrote code that inspected the
 signature of the `__init__()` method to set the attribute names
 in a `_fields` class variable.   For example:
 
@@ -108,7 +108,7 @@ methods on the `Structure` class--that approach was kind of weird.
 
 ## (c) Named Tuples
 
-In link:ex2_1.html[Exercise 2.1], you experimented with `namedtuple` objects
+In [Exercise 2.1](ex2_1.md), you experimented with `namedtuple` objects
 in the `collections` module.  Just to refresh your memory, here is how
 they worked: