doc: services: device_mgmt: smp_groups: Correct smp img upload request

The specification of que CBOR request 'image upload request' field
'sha' was decalred as type (str) but is (byte str).
Just replaced the field type after verified with code here:
/subsys/mgmt/mcumgr/grp/img_mgmt/src/img_mgmt.c#L383

Signed-off-by: Mathis Raemy <mathis.raemy@gmail.com>