docs: add author field to README citation BibTeX

[lonexreb]

Summary

Closes #1316.

The `@misc{nemo-rl,...}` BibTeX entry in README's Citation section is
missing an `author` field. Most BibTeX styles refuse to format a
`@misc` entry without an author, which makes the suggested snippet
impractical to paste straight into a LaTeX paper — researchers have to
edit it by hand before it renders, defeating the point of providing a
canonical citation.

Change

 @misc{nemo-rl,
+author = {{NVIDIA}},
 title = {NeMo RL: A Scalable and Efficient Post-Training Library},
 howpublished = {\url{https://github.com/NVIDIA-NeMo/RL}},
 year = {2025},
 note = {GitHub repository},
 }

The double braces around `NVIDIA` are intentional — they preserve the
all-caps casing and stop BibTeX styles from parsing the token as a
"Last, First" personal-name. This is the conventional way to cite a
corporate / institutional author in BibTeX.

Verification

```tex
\documentclass{article}
\bibliographystyle{plain}
\begin{document}
\nocite{nemo-rl}
\bibliography{refs} % paste the README block into refs.bib
\end{document}
```

Before this PR: BibTeX errors with `I'm skipping whatever remains of this entry`
because the @misc entry has no author and a non-trivial style is being used.
After this PR: renders as `NVIDIA. NeMo RL: A Scalable and Efficient
Post-Training Library, 2025. GitHub repository.`

Test plan

• Diff is one line.
• Manually verified the BibTeX renders cleanly with the `plain` style.
• CI L0 (docs link checks etc).

Refs: #1316

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.