Fix `write` return type in sink concept.

Thanks to Timothy Madden:
http://lists.boost.org/boost-docs/2010/09/4191.php

[SVN r66297]

libs/iostreams

@@ -1 +1 @@
-Subproject commit 3a5d223bdb6953b18166f39299381d92d9512a82
+Subproject commit 909de9c74a266101e6025ef2b6afc857b01a46b8