qmp-commands.hx: s/tray-open/tray_open/ to match qapi schema

Currently, we are using 'tray_open' in QMP and 'tray-open' in
HMP. However, the QMP documentation was mistakenly using the
HMP version.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qmp-commands.hx

@@ -1659,7 +1659,7 @@ Each json-object contain the following:
          - Possible values: "unknown"
 - "removable": true if the device is removable, false otherwise (json-bool)
 - "locked": true if the device is locked, false otherwise (json-bool)
-- "tray-open": only present if removable, true if the device has a tray,
+- "tray_open": only present if removable, true if the device has a tray,
                and it is open (json-bool)
 - "inserted": only present if the device is inserted, it is a json-object
    containing the following: