samples: subsys: shell: devmem_load: fix incorrect use of OVERLAY_CONFIG

Replace OVERLAY_CONFIG with CONF_FILE since the file in question
contains an alternative configuration to the one in prj.conf
as clearly stated in the sample doc
'If you use poll you should also use `prj_poll.conf` instead of
`prj.conf`.'

Signed-off-by: Abderrahmane Jarmouni <git@jarmouni.me>