Fix the code block display in github

It seems that github doesn't support indent by tabs in the code block,
so the rendering HTML is not so good.

Please compare [here][1] & [here][2].

[1]: https://github.com/sayuan/redmine-wiki_graphviz_plugin#graphviz_link-macro
[2]: https://github.com/tckz/redmine-wiki_graphviz_plugin#graphviz_link-macro