Update cudnn version in instruction for mac gpu. (#6407)

tensorflow/g3doc/get_started/os_setup.md

@@ -740,7 +740,7 @@ export PATH="$CUDA_HOME/bin:$PATH"
 ```
 
 Finally, you will also want to install the [CUDA Deep Neural
-Network](https://developer.nvidia.com/cudnn) (cuDNN v5) library which currently
+Network](https://developer.nvidia.com/cudnn) (cuDNN v5.1) library which currently
 requires an [Accelerated Computing Developer
 Program](https://developer.nvidia.com/accelerated-computing-developer) account.
 Once you have it downloaded locally, you can unzip and move the header and