A compound of formula (I) or a salt thereof wherein R1 represents a hydrogen atom, a methyl group or a ethyl group R2 represents a hydrogen atom or a fluorine atom R3 represents a hydrogen atom, a methyl group or an ethyl group.本發明係關於一種式(I)化合物或其鹽:R1表示氫原子、甲基或乙基;R2表示氫原子或氟原子;R3表示氫原子、甲基或乙基。