[TTS] [bugfix] German tutorial directory correction
Signed-off-by: Akshit Arora <akshit.arora@colorado.edu>

# What does this PR do ?

Corrected the directory created to download validation_ds, required for HiFi-GAN fine-tuning.

**Collection**: TTS

# Changelog 
From 
```
!cd NeMoGermanTTS && mkdir -p model/train_ds && cd model/validation_ds && wget https://raw.githubusercontent.com/nvidia/NeMo/$BRANCH/examples/tts/conf/hifigan/model/validation_ds/val_ds_finetune.yaml
```
to:
```
!cd NeMoGermanTTS && mkdir -p model/validation_ds && cd model/validation_ds && wget https://raw.githubusercontent.com/nvidia/NeMo/$BRANCH/examples/tts/conf/hifigan/model/validation_ds/val_ds_finetune.yaml
```

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