Update Tacotron2's list_available_models()
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Tacotron2's `list_available_models()` was still pointing to a previous version, so it would load that instead of the newest one.

**Collection**: TTS

**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

# Additional Information
* As noticed in #5714 
