qemu-options.hx: Update for reboot-timeout parameter

Since ee5d0f89, -1 is not valid for the value of reboot-timeout. Update
that in qemu-options doc.
Signed-off-by: NHan Han <hhan@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191015151451.727323-1-hhan@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

qemu-options.hx

@@ -327,8 +327,8 @@ format(true color). The resolution should be supported by the SVGA mode, so
 the recommended is 320x240, 640x480, 800x640.
 
 A timeout could be passed to bios, guest will pause for @var{rb_timeout} ms
-when boot failed, then reboot. If @var{rb_timeout} is '-1', guest will not
-reboot, qemu passes '-1' to bios by default. Currently Seabios for X86
+when boot failed, then reboot. If @option{reboot-timeout} is not set,
+guest will not reboot by default. Currently Seabios for X86
 system support it.
 
 Do strict boot via @option{strict=on} as far as firmware/BIOS