[boost] [filesystem] two-minute tutorial uses deprecated functions

The two-minute tutorial section of the filesystem documentation calls leaf() from a dereferenced directory_iterator. The reference documentation shows that dereferencing a directory_iterator yields a directory_entry but the reference documentation for directory_entry shows no leaf member. Eventually I had to track it down in the source code and found out that leaf does exist but is deprecated and not documented. It is probably bad to have the introduction to a library use undocumented functions. Thanks, Michael Marcin