[clang] [clang][ReleaseNotes] Fix code block not rendering (PR #135330)

[Mészáros Gergely via cfe-commits]

Maetveis wrote:

Before (https://clang.llvm.org/docs/ReleaseNotes.html#improvements-to-clang-s-diagnostics):

![image](https://github.com/user-attachments/assets/1f6c4d56-6d2b-41c8-985c-f5ec30461987)

With this PR:

![image](https://github.com/user-attachments/assets/19a017e3-44fb-4179-accd-f2a3ca249ed5)

https://github.com/llvm/llvm-project/pull/135330