Updated quick-start example with BertForMaskedLM

As `convert_ids_to_tokens` returns a list, the code in the README currently throws an `AssertionError`, so I propose I quick fix.

README.md

@@ -142,7 +142,7 @@ predictions = model(tokens_tensor, segments_tensors)
 
 # confirm we were able to predict 'henson'
 predicted_index = torch.argmax(predictions[0, masked_index]).item()
-predicted_token = tokenizer.convert_ids_to_tokens([predicted_index])
+predicted_token = tokenizer.convert_ids_to_tokens([predicted_index])[0]
 assert predicted_token == 'henson'
 ```