Update NeMo_TTS_Primer.ipynb
# What does this PR do ?

Fix a mistake in line 782. FFT calculates the magnitude and phase of each frequency bin not frequency band. Moreover, frequency bins in FFT are not related to pitch. So, we should change _**band (ie. pitch)**_ to _**bin**_. 

**Collection**: [TTS]

# Changelog 
tutorials/tts/NeMo_TTS_Primer.ipynb
  
**PR Type**:
- [ ] New Feature
- [ ] Bugfix
- [x] Documentation

## Who can review?
@blisc & @okuchaiev
