[handbook] Change \Cplusplus and get rid of package relsize.

Relsize is not part of standard TeX live and causes errors.
Use the command suggested from the C++ FAQ.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14078 2fb0f335-1f38-0410-981e-8018bf24f1b0