[clang-tidy] Use literal block instead of code block (NFC)

I used a C++ code block in check documentation to show example
output from clang-tidy, but since the example output isn't
kosher C++, sphinx didn't like that when it went to syntax
highlight the block.  So switch to a literal block instead
and forego any highlighting.

Fixes build error
<https://lab.llvm.org/buildbot/#/builders/115/builds/21145>