fix typo

fix typo

mrcnn/model.py

@@ -2253,7 +2253,7 @@ class MaskRCNN():
         layers: Allows selecting wich layers to train. It can be:
             - A regular expression to match layer names to train
             - One of these predefined values:
-              heaads: The RPN, classifier and mask heads of the network
+              heads: The RPN, classifier and mask heads of the network
               all: All the layers
               3+: Train Resnet stage 3 and up
               4+: Train Resnet stage 4 and up