[TTS] Fix dataset parameter typo on tacotron2 example yaml
# What does this PR do ?

This PR fixes dataset parameter typo on tacotron2 example yaml

**Collection**: [TTS]

# Changelog 
- Change "train_dataset" to "validation_datasets" on tacotron2.yaml conf example.

# 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)
- [ ] Did you write any new necessary tests? -> I checked that locally
- [ ] Did you add or update any necessary documentation? -> No documentation needed
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc) -> It does not
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] Bugfix

## 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
* Related to # (issue)
