NFA samples fix
# What does this PR do ?

Correct NFA README typo and fix an NFA error that would give incorrect timestamps when the input audio had a different sampling rate to that of the ASR model.

**Collection**: tools/nemo_forced_aligner

# Changelog 
- Correct NFA README typo: 'samples' -> 'seconds'
- Fix timestamps bug: change variable 'audio_sr' -> 'model.cfg.sample_rate'

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ N/A ] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
- [ N/A ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ N/A ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Addressing question raised in https://github.com/NVIDIA/NeMo/discussions/5848. 
