Fix title consistency from "Transformer Reinforcement Learning" to "Transformers Reinforcement Learning" (#5183)

Author: qgallouedec

README.md

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-# TRL - Transformer Reinforcement Learning
+# TRL - Transformers Reinforcement Learning
 
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docs/source/index.md

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-# TRL - Transformer Reinforcement Learning
+# TRL - Transformers Reinforcement Learning
 
 TRL is a full stack library where we provide a set of tools to train transformer language models with methods like Supervised Fine-Tuning (SFT), Group Relative Policy Optimization (GRPO), Direct Preference Optimization (DPO), Reward Modeling, and more.
 The library is integrated with 🤗 [transformers](https://github.com/huggingface/transformers).