AndוְI will putאָשִׁיתenmityאֵיבָהbetween youבֵּינְךָand the woman,וּבֵין הָאִשָּׁהand between your offspringוּבֵין זַרְעֲךָand her offspring;וּבֵין זַרְעָהּheהוּאwill strikeיְשׁוּפְyouךָon thehead,רֹאשׁandוְyouאַתָּהwill strikeתְּשׁוּפֶhimנּוּon theheel.”עָקֵב
Literally
“Offspring” translates zera, “seed,” a singular noun that can designate one or many (compare 22:17). The pronoun that follows, “he” (hu), is masculine singular and agrees with zera.
Source: Genesis 3:15; Genesis 22:17
Documented fact