Change required cuDNN from v5.1 to v6.0 for Linux in the docs (tensorflow#12463)

This fix changes the required cuDNN from v5.1 to v6.0 for Linux in
`tensorflow/docs_src/install/install_linux.md` as the build is
linked against libcudnn.so.6 (not libcudnn.so.5)

This fix fixed 12416.

Signed-off-by: Yong Tang <[email protected]>

Author: yongtang

tensorflow/docs_src/install/install_linux.md

@@ -33,7 +33,7 @@ must be installed on your system:
     `LD_LIBRARY_PATH` environment variable as described in the
     NVIDIA documentation.
   * The NVIDIA drivers associated with CUDA Toolkit 8.0.
-  * cuDNN v5.1. For details, see
+  * cuDNN v6.0. For details, see
     [NVIDIA's documentation](https://developer.nvidia.com/cudnn).
     Ensure that you create the `CUDA_HOME` environment variable as
     described in the NVIDIA documentation.