fixed delete

Author: nagano564

min_heap/min_heap_spec.rb

@@ -81,7 +81,7 @@
       tree.insert(root, node5) #86
       tree.insert(root, node11) #98
       tree.delete(root, node11.rating)
-      expect(tree.find(root, node5.rating)).to be_nil
+      expect(tree.find(root, node11.rating)).to be_nil
     end
   end