[Bug plugins/69928] incorrect reference to gcc-plugin.h in plugin documentation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69928

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It is installed at:
./lib/gcc/${target_triplet}/${version}/plugin/include/gcc-plugin.h