Fix cytokine to protein typo in ex1 visualize_diffusion config

Substance is named "protein" but visualize_diffusion referenced "cytokine",
causing a segfault in ParallelVtiWriter when export_visualization is enabled.

Author: sportokalidis

demo/regulatory_networks/src/bdm_ex1.h

@@ -104,7 +104,7 @@ inline int Simulate(int argc, const char** argv) {
     param->statistics = false;
     param->simulation_time_step = 1.0;
     param->visualize_diffusion = {
-        Param::VisualizeDiffusion{"cytokine", true, true}};
+        Param::VisualizeDiffusion{"protein", true, true}};
     param->calculate_gradients = false;
     param->diffusion_method = "euler";
   };