[doc] Fix pressure BC documentation.

.../benchmarks/small-deformations/Pressure_BC/Pressure_BC.md

...arks/small-deformations/Pressure_BC/pipe_axisymmetric.pdf

...arks/small-deformations/Pressure_BC/pipe_axisymmetric.png

...arks/small-deformations/Pressure_BC/pipe_plane_strain.pdf

...arks/small-deformations/Pressure_BC/pipe_plane_strain.png

...s/benchmarks/small-deformations/Pressure_BC/sphere_3d.pdf

...s/benchmarks/small-deformations/Pressure_BC/sphere_3d.png

...ks/small-deformations/Pressure_BC/sphere_axisymmetric.pdf

...ks/small-deformations/Pressure_BC/sphere_axisymmetric.png

...small-deformations/Pressure_BC/sphere_axisymmetric_pl.pdf

...small-deformations/Pressure_BC/sphere_axisymmetric_pl.png

...ns/Pressure_BC/sphere_axisymmetric_pl_residual_stress.pdf

...ns/Pressure_BC/sphere_axisymmetric_pl_residual_stress.png