Remove warning on build caused by non-existent pygment

At #10857 a gitignore codeblock was added. Addition seems correct,
but a pygment formatting was defined as:

.. code-block:: gitignore

No such a pygment exists, and although there is an aditional
package that could parse git-related files, we chose to
not add an extra package dependency for building the docs.

Fallback to unixconfig instead.

Closes #10989

tutorials/best_practices/version_control_systems.rst

@@ -130,7 +130,7 @@ order to access the LFS files.
 Below is an example ``.gitattributes`` file that you can use as a starting point for Git LFS. 
 These file types were chosen because they are commonly used, but you can modify the list to include any binary types you may have in your project.
 
-.. code-block:: gitignore
+.. code-block:: unixconfig
 
     # Normalize EOL for all files that Git considers text files.
     * text=auto eol=lf