Update Tacotron2 NGC checkpoint load to latest version
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Second version of #5748, applied to r1.15.0 this time.
Tacotron2's `list_available_models()` was still pointing to a previous version, so it would load that instead of the newest one.
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [x] Documentation

# Additional Information
* First noticed in #5714 
