Fix links in pre-requesites section

doc/developer/scan.txt

@@ -32,11 +32,9 @@ Pre-requisites
 
 The following sections assumes the reader is familiar with the following :
 
-1. Theano's graph structure (Apply nodes, Variable nodes and Ops)
-    http://deeplearning.net/software/theano/tutorial/extending_theano.html#theano-graphs
+1. Theano's :ref:`graph structure <extending_theano>` (Apply nodes, Variable nodes and Ops)
 
-2. The interface and usage of Theano's `scan()` function
-    http://deeplearning.net/software/theano/library/scan.html
+2. The interface and usage of Theano's :ref:`scan() <lib_scan>` function
 
 
 Relevant code files