Fix logic error in an assertion

The CI system found this for me, because it only goes wrong
when dune-foamgrid is not available.

src/harmonicmaps.cc

@@ -133,7 +133,7 @@ int main (int argc, char *argv[])
 #if HAVE_DUNE_FOAMGRID
     typedef std::conditional<dim==1 or dim!=dimworld,FoamGrid<dim,dimworld>,UGGrid<dim> >::type GridType;
 #else
-    static_assert(dim!=dimworld, "You need to have dune-foamgrid installed for dim != dimworld!");
+    static_assert(dim==dimworld, "You need to have dune-foamgrid installed for dim != dimworld!");
     typedef std::conditional<dim==1,OneDGrid<dim>,UGGrid<dim> >::type GridType;
 #endif