Update README.md (#51)

fixed typo

README.md

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 ## What is it?
-With `trl` you can train transformer language models with Proximal Policy Optimization (PPO). The library is built on top of the [`transformer`](https://github.com/huggingface/transformers) library by  🤗 Hugging Face. Therefore, pre-trained language models can be directly loaded via `transformers`. At this point only decoder architectures such as GTP2 are implemented.
+With `trl` you can train transformer language models with Proximal Policy Optimization (PPO). The library is built on top of the [`transformer`](https://github.com/huggingface/transformers) library by  🤗 Hugging Face. Therefore, pre-trained language models can be directly loaded via `transformers`. At this point only decoder architectures such as GPT2 are implemented.
 
 **Highlights:**
 - PPOTrainer: A PPO trainer for language models that just needs (query, response, reward) triplets to optimise the language model.