Doc: Replace the deprecated highlightlang directive by highlight. (#13377)

highlightlang is deprecated since April 2018 in Sphinx.
See https://github.com/sphinx-doc/sphinx/pull/4845
diff --git a/Doc/using/unix.rst b/Doc/using/unix.rst
index 31cf87c..021f0d3 100644
--- a/Doc/using/unix.rst
+++ b/Doc/using/unix.rst
@@ -1,4 +1,4 @@
-.. highlightlang:: sh
+.. highlight:: sh
 
 .. _using-on-unix: