deprecated.hh: fix typo

although, if you're unlucky, it worked anyway because the DUNE_DEPRECATED
macro might get defined before dumux/common/deprecated.hh gets included. that
would be pure unluck, though. I _love_ the autotools.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7849 2fb0f335-1f38-0410-981e-8018bf24f1b0