Speculatively fix the sphinx build for Clang's attribute reference

This is a cherry-pick of a10ff37

Author: AaronBallman

clang/include/clang/Basic/AttrDocs.td

@@ -432,7 +432,7 @@ implementation detail and not intended to be used by external users.
 
 The syntax of the attribute is as follows:
 
-.. code-block:: c++
+.. code-block:: text
 
   class __attribute__((sycl_special_class)) accessor {};
   class [[clang::sycl_special_class]] accessor {};