Fix sphinx build for clang-tools-extra

I think it doesn't like the non-ASCII characters in the block, so using
a text block to disable syntax highlighting.

This should fix:
https://lab.llvm.org/buildbot/#/builders/115/builds/29888

clang-tools-extra/docs/clang-tidy/checks/misc/confusable-identifiers.rst

@@ -9,7 +9,7 @@ attack described in `CVE-2021-42574 <https://www.cve.org/CVERecord?id=CVE-2021-4
 
 Example:
 
-.. code-block:: c++
+.. code-block:: text
 
     int fo; // Initial character is U+0066 (LATIN SMALL LETTER F).
     int 𝐟o; // Initial character is U+1234 (SUPER COOL AWESOME UPPERCASE NOT LATIN F) not U+0066 (LATIN SMALL LETTER F).