[handbook][pics] Apply renaming of png folder

doc/handbook/5_spatialdiscretizations.tex

@@ -220,7 +220,7 @@ with the tensor $\mathbf{\Lambda}_K$ associated with control volume $K$, the dis
 
 \begin{figure} [ht]
 \centering
-\includegraphics[width=0.4\linewidth,keepaspectratio]{PNG/cctpfa.png}
+\includegraphics[width=0.4\linewidth,keepaspectratio]{png/cctpfa.png}
 \caption{Two neighboring control volumes sharing the face $\sigma$.}
 \label{pc:cctpfa}
 \end{figure}
@@ -271,7 +271,7 @@ Using these conditions, the intermediate face unknowns ${u}_\sigma$ can be elimi
 
 \begin{figure} [ht]
 \centering
-\includegraphics[width=0.8\linewidth,keepaspectratio]{PNG/mpfa_iv.png}
+\includegraphics[width=0.8\linewidth,keepaspectratio]{png/mpfa_iv.png}
 \caption{Interaction region for the Mpfa-O method. The graphic on the right illustrates how the sub-control volume $L^v$ and face $\sigma^v_2$ are embedded in cell $L$. Note that the face stencils for all sub-control volume faces in the depicted interaction region are $\mathcal{S}_{\sigma^v_i} = \{ K,L,M \}$, meaning that the fluxes over the sub-control volume faces depend on the three cell unknowns $u_K, u_L, u_M$.}
 \label{pc:interactionRegion_mpfa}
 \end{figure}