Fix Clang sphinx build

This addresses the issue found by:
https://lab.llvm.org/buildbot/#/builders/92/builds/36449

Author: AaronBallman

clang/docs/ClangRepl.rst

@@ -21,7 +21,7 @@ Clang-Repl Usage
 ================
 
 
-.. code-block:: c++
+.. code-block:: text
 
   clang-repl> #include <iostream>
   clang-repl> int f() { std::cout << "Hello Interpreted World!\n"; return 0; }