Correct titles underlines

Author: luisaFelixSalles

doc/source/user_guide/tutorials/custom_operators_and_plugins/custom_plug_in_package_third_deps.rst

@@ -144,7 +144,7 @@ Use the :func:`load_library() <ansys.dpf.core.core.load_library>` method to load
     "load_operators")
 
 Use the custom operators
---------------------
+------------------------
 
 Once the plugin is loaded, you can instantiate the custom operator:
 

doc/source/user_guide/tutorials/mesh/extract_mesh_in_split_parts.rst

@@ -90,7 +90,7 @@ The split meshes are given in a |MeshesContainer| and can be spatially or tempor
             print(meshes_41)
 
 Scope the mesh regions to be extracted in split regions
------------------------------------------------------
+-------------------------------------------------------
 
 A region corresponds to a zone for Fluid and CFX results. You can specify the mesh regions you want to get by giving
 the zones ids to the ``region_scoping`` argument.