[doc] Fix pressure BC documentation.

Use png for images. Fix sections in the markdown.

web/content/docs/benchmarks/small-deformations/Pressure_BC/Pressure_BC.md

@@ -10,28 +10,28 @@ weight = 117
 
 +++
 
-##Problem description
+## Problem description
 
 Five different benchmarks are reported in which pressure boundary conditions are tested: an axisymmetric elastic cylinder, a plain strain elastic cylinder, an axisymmetric elastic sphere, a tri-dimensional elastic sphere and an axisymmetric elasto-plastic sphere.
 See [this PDF](../pressure_bc.pdf) for detailed problem description.
 
-##Results and evaluation
+## Results and evaluation
 
 Plain strain elastic cylinder comparison between numerical and analytical results:
-{{< img src="../pipe_plane_strain.pdf" >}}
+{{< img src="../pipe_plane_strain.png" >}}
 
 Axisymmetric elastic cylinder comparison between numerical and analytical results.
-{{< img src="../pipe_axisymmetric.pdf" >}}
+{{< img src="../pipe_axisymmetric.png" >}}
 
 Axisymmetric elastic sphere comparison between numerical and analytical results:
-{{< img src="../sphere_axisymmetric.pdf" >}}
+{{< img src="../sphere_axisymmetric.png" >}}
 
 Tri-dimensional elastic sphere comparison between numerical and analytical results:
-{{< img src="../sphere_3d.pdf" >}}
+{{< img src="../sphere_3d.png" >}}
 
 Axisymmetric plastic sphere comparison between numerical and analytical results:
-{{< img src="../sphere_axisymmetric_pl.pdf" >}}
+{{< img src="../sphere_axisymmetric_pl.png" >}}
 
 Axisymmetric plastic sphere residuals of stress:
-{{< img src="../sphere_axisymmetric_pl_residual_stress.pdf" >}}
+{{< img src="../sphere_axisymmetric_pl_residual_stress.png" >}}