Fix broken links

Exercises/ex6_3.md

@@ -63,7 +63,7 @@ mappingproxy(OrderedDict([('x', <Parameter "x">), ('y', <Parameter "y">)]))
 
 ## (c) Putting it Together
 
-In link:ex6_1.html[Exercise 6.1], you created a class `Structure`
+In [Exercise 6.1](ex6_1.md), you created a class `Structure`
 that defined a generalized `__init__()`, `__setattr__()`, and `__repr__()`
 method.   That class required a user to define a `_fields` class
 variable like this:
@@ -75,7 +75,7 @@ class Stock(Structure):
 
 The problem with this class is that the `__init__()` function didn't
 have a useful argument signature for the purposes of help and
-keyword argument passing.  In link:ex6_2.html[Exercise 6.2], you 
+keyword argument passing.  In [Exercise 6.2](ex6_2.md), you 
 did a sneaky trick involving a special `self._init()` function. For example:
 
 ```python