Update url. fix gh-3657

doc/install.txt

@@ -484,7 +484,7 @@ Using the GPU
 
 The first thing you'll need for Theano to use your GPU is Nvidia's
 GPU-programming toolchain. You should install at least the CUDA driver and the CUDA Toolkit, as
-`described here <http://www.nvidia.com/object/cuda_get.html>`_.  The CUDA
+`described here <https://developer.nvidia.com/cuda-toolkit>`_.  The CUDA
 Toolkit installs a folder on your computer with subfolders *bin*, *lib*,
 *include*, and some more too.  (Sanity check: The *bin* subfolder should contain an *nvcc*
 program which is the compiler for GPU code.)  This folder is called the *cuda