qga/qapi-schema: Fix member documentation markup

GuestDiskStatsInfo's member documentation is parsed as ordinary text
due to missing colons.  The generated documentation shows these
members as "Not documented".

The fix is obvious: add the missing colons.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230425064223.820979-12-armbru@redhat.com>

qga/qapi-schema.json

@@ -1553,11 +1553,11 @@
 ##
 # @GuestDiskStatsInfo:
 #
-# @name disk name
+# @name: disk name
 #
-# @major major device number of disk
+# @major: major device number of disk
 #
-# @minor minor device number of disk
+# @minor: minor device number of disk
 ##
 { 'struct': 'GuestDiskStatsInfo',
   'data': {'name': 'str',