Fix syntax in Remarkup Code Blocks docs
Summary:
Render the mentioned "two spaces" and "three backticks" to make examples understandable.
Fixes Q198
Test Plan:
* Go to http://phorge.localhost/reference/remarkup/#code-blocks
* Go to http://phorge.localhost/book/phorge/article/remarkup/#code-blocks
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26270