docs/misra/rules.rst: Fix entry for 20.12 rule

Commit 4cac80e226 broke the CI cppcheck jobs by adding an entry for
a rule 20.12 with "Severity" and "Summary" fields placed in reverse order.
This leads to an error as reported by convert_misra_doc.py:
No summary for rule 20.12

Fixes: 4cac80e226 ("docs/misra/rules.rst: add rule 16.6 and 20.12")
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>

docs/misra/rules.rst

@@ -561,10 +561,10 @@ maintainers if you want to suggest a change.
      -
 
    * - `Rule 20.12 <https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_20_12.c>`_
+     - Required
      - A macro parameter used as an operand to the # or ## operators,
        which is itself subject to further macro replacement, shall only
        be used as an operand to these operators
-     - Required
      - Variadic macros are allowed to violate the rule.
 
    * - `Rule 20.13 <https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_20_13.c>`_