documentation link bug

[tom fogal]

In the current documentation for derived filebufs:

  http://gcc.gnu.org/onlinedocs/libstdc++/manual/ext_io.html

A link named "__gnu_cxx::stdio_filebuf" (in the last bullet) instead
refers to the reference for "__gnu_cxx::character":

  http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a00063.html

AFAICT, it should refer to:

  http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a00087.html

Cheers,

-tom