A compound of the formula (I) or a salt thereof, wherein R1 represents a hydrogen atom, a methyl group or an ethyl group, R2 represents a hydrogen atom or a fluorine atom, and R3 represents a hydrogen atom, a methyl group or an ethyl group. [Selection figure] None【化1】 R1が水素原子、メチル基又はエチル基を表し、R2が水素原子又はフッ素原子を表し、R3が水素原子、メチル基又はエチル基を表す、式(I)の化合物又はその塩。【選択図】なし