Fix code block rendering issue in adding_an_op.md (#18368)

* Fix code block rendering issue in adding_an_op.md

In adding_an_op.md, html code was used in markdown for code blocks.
However, this does not work very well as some of the code blocks
includes incorrect rendering.

This fix converts html into "```c++" (backticks) so that the rendering
could be fixed.

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

* Fix additional html code

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

* Fix lang-cpp issue

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

* Further clean up

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