fix: [stix2 export] Fixed custom object type
fix: [stix2 export] Fixed custom object type

- Custom Object type cannot accept capital letters

FloatingGhost/MISP,SteveClement/MISP,Rafiot/MISP,MISP/MISP,MISP/MISP,FIRSTdotorg/MISP,Rafiot/MISP,devnull-/MISP,SteveClement/MISP,MISP/MISP,FloatingGhost/MISP,FloatingGhost/MISP,FIRSTdotorg/MISP,devnull-/MISP,SteveClement/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,Rafiot/MISP,SteveClement/MISP,devnull-/MISP,MISP/MISP,FIRSTdotorg/MISP,SteveClement/MISP,devnull-/MISP,Rafiot/MISP,MISP/MISP