[clang] Update OpenMPSupport.rst (PR #122174)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

```Warning, treated as error:
/home/runner/work/llvm-project/llvm-project/clang-build/tools/clang/docs/OpenMPSupport.rst:398:Inline interpreted text or phrase reference start-string without end-string.
ninja: build stopped: subcommand failed.
```
>From the CI

https://github.com/llvm/llvm-project/pull/122174