Update README.md

Moved the second picture up from overview section

lectures/17_trees_I/README.md

@@ -97,6 +97,8 @@ public:
 
 - Note: Sometimes a 3rd pointer — to the parent TreeNode — is added.
 
+  ![alt text](ds_set_diagram.png "ds set diagram")
+
 ## 17.6 Exercises
 
 1. Write a templated function to find the smallest value stored in a binary search tree whose root node is pointed
@@ -128,8 +130,6 @@ node) that does all of the work.
 - Because the class stores and manages dynamically allocated memory, a copy constructor, operator=, and
 destructor must be provided.
 
-![alt text](ds_set_diagram.png "ds set diagram")
-
 ## 17.9 Exercises
 
 1. Provide the implementation of the member function ds_set<T>::begin. This is essentially the problem of