Tony Crumpton, CubsHQ.com:
The final pitch that was a slider was a clear ball out of the strike zone, but the umpire made the game-ending call to the shocked faces of the Cubs players especially Zobrist.
The call looked pretty bad on TV, but it wasn't all that aggregious according to the on-screen strike zone.
And according to Brooks Baseball, it might actually have been a strike:
Comparing the two zones, the TV zone appears smaller than the actual strike zone.