Prompt learning notebook bugfix
# What does this PR do ?
Fixes notebook typo, changes branch to r1.9.0, improves manual dataset download message

**Collection**: BigNLP

# Changelog 
- Prompt Learning Notebook

# Usage
N/A

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**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
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- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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# Additional Information
* Related to https://nvbugs/3639143 [ML-NeMo][A30] Multitask_Prompt_and_PTuning.ipynb branch name not 'r1.9.0' and failed with ImportError: cannot import name 'VirtualPromptStyle' as shown below if manually changed to the r1.9.0 branch