[Bug - Outdated Docs] Docs still direct users to use specialized branch for Nemotron-3-Nano training, rather than main

**Describe the bug**

These docs direct users to a branch for Nano-v3:
https://github.com/NVIDIA-NeMo/RL/blob/main/docs/guides/nemotron-3-nano.md

But support was added to main here: https://github.com/NVIDIA-NeMo/RL/pull/1989
