qapi/accelerator: Fix markup of heading

qapi/accelerator: Fix markup of heading

The docs generated for qapi/accelerator.json shows text "=
Accelerators" instead of a heading.  This is because the patch that
added the heading crossed with the commit that changed heading
markup (commit 6c10778826a "docs/sphinx: remove special parsing for
freeform sections").  Fix the markup.

Fixes: 18da42ee4273 (qapi/accel: Move definitions related to accelerators in their own file)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250724091742.1950167-2-armbru@redhat.com>