Change required cuDNN from v5.1 to v6.0 for Linux in the docs (#12463)
Change required cuDNN from v5.1 to v6.0 for Linux in the docs (#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 <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
tensorflow/docs,tensorflow/docs-l10n,tensorflow/docs,tensorflow/docs