Well I don't think 1/(x+1) + 1 is right, but it's of the right form. It's probably because someone saw that x/(x+a) curve and thought it looked close enough to logarithmic without bothering to check it.
I suspect the linear misconception came from looking at the damage needed to damage dealt ratio, which is in fact linear in a small range:
But when you extend your view a bit you'll see that it is more of a piecewise made up of different linear growths:
So I'm much more willing to leave linear on the wiki rather than logarithmic. It may not be entirely accurate, but it's less confusing to the average reader.