Fix typo in training (#5510)

docs/source/training.rst

@@ -39,7 +39,7 @@ of the specified model are used to initialize the model. The
 library also includes a number of task-specific final layers or 'heads' whose
 weights are instantiated randomly when not present in the specified
 pre-trained model. For example, instantiating a model with
-``BertForSequenceClassification.from_pretrained('bert-base-uncased', num_classes=2)``
+``BertForSequenceClassification.from_pretrained('bert-base-uncased', num_labels=2)``
 will create a BERT model instance with encoder weights copied from the
 ``bert-base-uncased`` model and a randomly initialized sequence
 classification head on top of the encoder with an output size of 2. Models