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/arg.rst b/Doc/c-api/arg.rst
index b41130e..ba9ca5e 100644
--- a/Doc/c-api/arg.rst
+++ b/Doc/c-api/arg.rst
@@ -1,4 +1,4 @@
-.. highlightlang:: c
+.. highlight:: c
 
 .. _arg-parsing: