BTTF2 also predicted that the Cubs will win the World Series in 2015, in a game 7 sweep, against Miami.
At the time the movie was made, Miami didn't have a team. Today Miami does have an MLB team. So that prediction has come true.
Miami and Chicago Cubs are in the same league, so they can't face each other in the World Series. So at least this year, that part cannot come true.
Also, it's been debated what was meant by a game 7 sweep, as mentioned in the movie. If it's a sweep and they played to game 7, that implies it was a best of 11 series. While baseball is long today, I can't imagine even the world series being expanded to 11-game series. Thus, at least this year, this part also cannot come true. It could have meant that the Cubs were down 0-3, and then came back to win 4 games in a row and the series in game 7. I think traditionalists wouldn't call this a sweep though, more of a mega come-back (Sox 2004 ALCS!!)
All that said, it is still possible for the Cubs to win the World Series - and that prediction coming true alone would be quite an amazing coincidence. Although it's still early in the season, the Cubs are doing pretty well and at this rate might very well make the playoffs.