The word IS actually penance, and just another example of someone changing the Bible.
penance: Middle English, from Old French, from Latin paenitentia, penitence, from paenitēns, paenitent-, penitent; see penitent.
Repent:Middle English repenten, from Old French repentir : re-, re- + pentir, to be sorry (from Vulgar Latin *paenitīre, from Latin paenitēre).
Notice the change is very small but it still strays from the inspired Word of God as Canonized by the Catholic Church. Why, I don't know but maybe to support a belief that someone else decided to change???
Now, I know you don't like the Catholic religion but explain to me again how you accept any part of the New Testament as the inspired Word of God? Who was the authority that gave YOU the Bible?
The word is metanoiete, from meta, “change” and noos, “mind”; it is a verb in imperative tense. “Repent”, “do penance”, “change your mind”, “reform yourself” are all possible translations, except “change your mind” is an idiom that has a connotation of fickleness in the English language, even though it is the closest morphologically. If you look at the actions of St. John the Forerunner, you see similarity with penitential work of Christian monks, hence “do penance” becomes the most accurate on substance.
The controversy was, by the way, not about how people should repent but about the necessity of sacramental penance. Whether we translate St. Peter’s words as “repent” or as “do penance” does not change anything in the fact that in order to enter the Church one has to do a sacramental confession and be baptised.