Use https for `https://github.com/tensorflow/serving` (#16030)

* Use https for `https://github.com/tensorflow/serving`



It looks like there are a couple of links in saved_model.md
using http to point to serving, and the rest are using https
to point to serving, This fix updates to `https` for consistency
and security.

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