Update docs for installing CUDA/CUDNN (#16495)

* Update docs for installing CUDA/CUDNN

This fix addresses the issue raised in 16479 where
CUDA/CUDNN versions from the docs do not match TensorFlow v1.5.0.

From the Dockerfile and docker images ENV, the version of CUDA/CUDNN
for TensorFlow v1.5.0:
```
CUDA_VERSION 9.0.176
CUDNN_VERSION 7.0.5.15
```

This fix updates the doc so that CUDA version is changed from `8.0` -> `9.0`,
CUDNN version is changed from `6.0` -> `7.0`.

This fix fixes 16479.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>