[doc][handbook] more than one cc scheme is available now

doc/handbook/5_spatialdiscretizations.tex

 \section{Spatial Discretization Schemes}
 \label{spatialdiscretization}
 
-We discretize space with the cell-centered finite volume method (\ref{cc} ), the box method (\ref{box})
+We discretize space with cell-centered finite volume methods (\ref{cc} ), the box method (\ref{box})
 or a staggered grid scheme.
 Grid adaption is available for both box and cell-centered finite volume method.
 In general, the spatial  parameters, especially the porosity, have to be assigned on