C++ syntax highlighting does not work on multi-syntax code block

[victorrschmidt]

Bug description

The multi-syntax code block does not highlight C++ code, while still does highlights other languages.

How to reproduce

1. Create a multi-syntax code block on a gitbook page.
2. Choose the "C++" language option and write some piece of code.
3. Save the page and check it on the proper gitbook.io page.
4. The code will not be highlighted.