RooNLLVar documentation

Hello,

It looks like in the documentation of RooNLLVar the definition is not displayed correctly. Maybe it’s just my browser, but I tried a few different ones and every time it looks like the code is not properly interpreted in the documentation.

Best regards,
Jerzy

This PR, fixes this issue:

Once @moneta will have reviewed it we will push it tho the ROOT master doc.
Thanks to have seen it.