[Doc] Still using old MPLLIBS_STRING macro in the tutorial

[ldionne]

The tutorial is still using the MPLLIBS_STRING and MPLLIBS_DEFINE_ERROR macros instead of their BOOST_xxx counterpart. This should be fixed if the library makes it in.

[sabel83]

Yes, and it looks like the diagrams need to be updated as well. I'll fix these after the review (once the xxx part of the BOOST_xxx names have been chosen :))