Actually, that reading isn't found, but in modern bibles:
Here it is, from one of the many trusted, old-time, bibles:
"And when he was entered into the house from the people, his disciples asked him concerning the parable. And he saith unto them, Are ye so without understanding also? Do ye not perceive, that whatsoever thing from without entereth into the man, it cannot defile him; because it entereth not into his heart, but into the belly, and goeth out into the draught, purging all meats? And he said, That which cometh out of the man, that defileth the man. For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lasciviousness, an evil eye, blasphemy, pride, foolishness: all these evil things come from within, and defile the man." --Mark 7:17-23
As you can see, the difference is gross, leading many to conclude that all foods are clean: this also, more harmfully, creates an immediate internal contradiction in the bible, which can cause many to doubt their faith altogether.
So this is actually a big deal.