Corrected typo in sediment_mod.h

[arango]

Corrected a typo in ROMS/Nonlinear/Sediment/sediment_mod.h during the allocation of variable transC and transN. We need to have instead:

#if defined MIXED_BED
      IF (.not.allocated(transC)) THEN
        allocate ( transC(Ngrids) )
        transC = IniVal
      END IF
      IF (.not.allocated(transN)) THEN
        allocate ( transN(Ngrids) )
        transN = IniVal
      END IF
#endif

Many thanks to Alfredo Aretxabaleta for bringing this to my attention.