Add `model.train()` line to ReadMe training example
Add `model.train()` line to ReadMe training example

Co-Authored-By: Santosh-Gupta <f3a03b4610c506915b48485d3b08710ecac0cd98@gmail.com>

huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/pytorch-transformers,huggingface/transformers