Fix incorrect rendering in community/documentation.md

This fix tries to fix the incorrect rendering in community/documentation.md.
The issue was caused by the escaping of "```". As could be seen
in the docs, there are "```" (backticks) inside the code block and
the code block was surrended by "```" (backticks) as well. The
result is that backticks confused the inteprater of markdown and
messed the documentation.

This fix uses indent four spaces to wrap the code blocks so that
backticks inside could be rendered correctly.

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