Fix table markup to show markup table

[hugovk]

The table at https://devguide.python.org/documentation/markup/#quick-reference is missing:

The warning in the logs:

/Users/hugo/github/devguide/documentation/markup.rst:25: ERROR: Malformed table.
Text in column margin in table line 7.

======================= =========================================== ====================
Element                 Markup                                      See also
======================= =========================================== ====================
arguments/parameters    ``*arg*``                                   :ref:`inline-markup`
variables/literals/code ````foo````, ````42````, ````len(s) - 1```` :ref:`inline-markup`
True/False/None         ````True````, ````False````, ````None````   :ref:`inline-markup`
function definitions   ``.. function:: print(*args)``               :ref:`directives`
function references    ``:func:`print```                            :ref:`roles`
attribute definitions   ``.. attribute: `attr-name```               :ref:`information-units`
attribute references    ``:attr:`attr-name```                       :ref:`roles`
reference labels        ``.. _label-name:``                         :ref:`doc-ref-role`
internal references     ``:ref:`label-name```                       :ref:`doc-ref-role`
external links          ```Link text <https://example.com>`_``      :ref:`hyperlinks`
roles w/ custom text    ``:role:`custom text <target>```            :ref:`roles`
roles w/ only last part ``:role:`~hidden.hidden.visible```          :ref:`roles`
roles w/o link          ``:role:`!target```                         :ref:`roles`
issues                  ``:gh:`ID```, ``:issue:`ID```               :ref:`roles`
CPython source          ``:source:`PATH```                          :ref:`roles`
comments                ``.. a comment``                            :ref:`comments`
======================= =========================================== ==================== [docutils]
looking for now-outdated files... none found

https://github.com/python/devguide/actions/runs/13209380442/job/36879823502#step:5:94

It was accidentally broken in #1491 and not noticed because we temporarily disabled --fail-on-warning in #1426 as part of the re-org work to prevent errors from duplicate label warnings.

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📚 Documentation preview 📚: https://cpython-devguide--1511.org.readthedocs.build/