Fix typo in HiFi-GAN config's max steps
Signed-off-by: Jocelyn Huang <jocelynh@nvidia.com>

# What does this PR do ?

Fix typo in the default `max_steps` for HiFi-GAN configs (see #4431).

**Collection**: TTS

# Changelog 
- Changed default steps from 25M to 2.5M

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

# Additional Information
* Raised in discussion #4431
