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/installing/index.rst b/Doc/installing/index.rst
index 747b9223..dc44aa6 100644
--- a/Doc/installing/index.rst
+++ b/Doc/installing/index.rst
@@ -1,4 +1,4 @@
-.. highlightlang:: none
+.. highlight:: none
 
 .. _installing-index: