[Docs] Switch a code block from LLVM to text

While I can't replicate this locally, it looks like the buildbots don't
recognize the IR block in r358385 l764 as IR. Downgrade it to being just
text while I look into it.

http://lab.llvm.org:8011/builders/llvm-sphinx-docs/builds/30132

llvm-svn: 358391