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/bool.rst b/Doc/c-api/bool.rst
index a9fb342..ce8de6e 100644
--- a/Doc/c-api/bool.rst
+++ b/Doc/c-api/bool.rst
@@ -1,4 +1,4 @@
-.. highlightlang:: c
+.. highlight:: c
 
 .. _boolobjects: