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 6c10778826 "docs/sphinx: remove special parsing for
freeform sections").  Fix the markup.

Fixes: 18da42ee42 (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>

qapi/accelerator.json

@@ -4,7 +4,9 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 ##
-# = Accelerators
+# ************
+# Accelerators
+# ************
 ##
 
 { 'include': 'common.json' }