Update readme granular flow example

Author: ncrescenzio

mpm/validation/granular_flow_2D/README.md

@@ -34,11 +34,11 @@ The problem stated above has been solved with a structured mesh with 3 material
 
 <p align="center">
   <img src="data/granular_flow_2D_results.png" alt="Obtained results and comparison." width="700" />
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+</p>
+<p align="center">
   (a.) Experiment conducted by (Bui et al., 2008), (b.) comparison of final surface configuration and failure line, (c.) simulation results of (Bui et al., 2008) by using SPH method, (d.) simulation results obtained by implicit MPM method
 </p>
 
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 ## References
-- Bui, H. H., Fukagawa, R., Sako, K., & Ohno, S. (2008). Lagrangian meshfree particles method (SPH) for large deformation and failure flows of geomaterial using elastic-plastic soil constitutive model. International Journal for Numerical and Analytical Methods in Geomechanics, 32(12), 1537–1570. https://doi.org/10.1002/nag.688
-- Chandra, B., Larese, A., Iaconeta, I., Rossi, R., Wüchner, R. (2018). Soil-Structure Interaction Simulation of Landslides Impacting a Structure Using an Implicit Material Point Method. *Accepted for publication by Proceeding of the 2nd International Conference on The Material Point Method (MPM2019)*.
+- Bui, H. H., Fukagawa, R., Sako, K., & Ohno, S. (2008). **Lagrangian meshfree particles method (SPH) for large deformation and failure flows of geomaterial using elastic-plastic soil constitutive model**, *International Journal for Numerical and Analytical Methods in Geomechanics*, 32(12), 1537–1570. DOI: <a href="https://doi.org/10.1002/nag.688">10.1002/nag.688</a>.
+- Chandra, B., Larese, A., Iaconeta, I., Rossi, R., Wüchner, R. (2018). **Soil-Structure Interaction Simulation of Landslides Impacting a Structure Using an Implicit Material Point Method**, *Accepted for publication by Proceeding of the 2nd International Conference on The Material Point Method (MPM2019)*.