doc: matrix.json not valid JSON causing warning/error

[NeroBurner]

Warning, treated as error:
/builds/hunter/hunter/docs/creating-new/test/ci.rst:84:Could not lex literal_block as "JSON". Highlighting skipped.

The referenced codeblock is the following:

.. literalinclude:: ../../../.github/workflows/ci/matrix.json
  :language: JSON

The matrix.json file contains // comments, which are not valid json tripping the highlighting.

Removing the :language: JSON part fixes this warning