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/c-api/marshal.rst b/Doc/c-api/marshal.rst
index 17ec621..b086830 100644
--- a/Doc/c-api/marshal.rst
+++ b/Doc/c-api/marshal.rst
@@ -1,4 +1,4 @@
-.. highlightlang:: c
+.. highlight:: c
 
 .. _marshalling-utils: