docs: Fix Sphinx error on non-YAML code snippet

This problem was just introduced by commit cec4f33 "Clarify disable-line
and parser errors, workaround" and produced this error when building
documentation:

    docs/disable_with_comments.rst:120:Could not lex literal_block as
    "yaml". Highlighting skipped.

docs/disable_with_comments.rst

@@ -117,7 +117,7 @@ post-template processing).
 Example of a Jinja2 code that cannot be parsed as YAML because it contains
 invalid tokens ``{%`` and ``%}``:
 
-.. code-block:: yaml
+.. code-block::
 
  # This file IS NOT valid YAML and will procuce syntax errors
  {% if extra_info %}