Fix Clang Sphinx bot

Addresses the issue found in:
https://lab.llvm.org/buildbot/#/builders/92/builds/48490

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -74,6 +74,7 @@ C++2c Feature Support
   is not supported.
 
   .. code-block:: cpp
+
     struct S {
       int _, _; // Was invalid, now OK
     };