add type

slides/cours_15.md

@@ -922,7 +922,7 @@ ajout(arbre, clé)
 tree_t position(tree_t tree, key_t key) {
     tree_t current = tree;
     if (NULL != current) {
-        subtree = key > current->key ? current->right :
+        tree_t subtree = key > current->key ? current->right :
         current->left;
         while (key != current->key && NULL != subtree) {
             current = subtree;