Add opening base lib in cxx-in-lua.rst
Add opening base lib in cxx-in-lua.rst

Loading `sol::lib::base` is needed since the corresponding lua file uses
`print` and `assert`.

OrfeasZ/sol2,OrfeasZ/sol2,ThePhD/sol2,ThePhD/sol2,OrfeasZ/sol2,OrfeasZ/sol2,ThePhD/sol2,ThePhD/sol2