Update the description of TverskyLoss (#626)

Author: gracikk-ds

segmentation_models_pytorch/losses/tversky.py

@@ -10,7 +10,7 @@
 
 class TverskyLoss(DiceLoss):
     """Tversky loss for image segmentation task.
-    Where TP and FP is weighted by alpha and beta params.
+    Where FP and FN is weighted by alpha and beta params.
     With alpha == beta == 0.5, this loss becomes equal DiceLoss.
     It supports binary, multiclass and multilabel cases