[restapi] fix python version during restapi startup (sonic-net#7056)

changed from python3 to python in supervisord.conf.

Author: kakkotetsu

dockers/docker-sonic-restapi/supervisord.conf

@@ -4,7 +4,7 @@ logfile_backups=2
 nodaemon=true
 
 [eventlistener:dependent-startup]
-command=python3 -m supervisord_dependent_startup
+command=python -m supervisord_dependent_startup
 autostart=true
 autorestart=unexpected
 startretries=0