[TTS] remove curly braces from ${BRANCH} in jupyer notebook cell.
# What does this PR do ?

**Change 1**
In jupyter notebook, when trying to run,
```
## Install NeMo
BRANCH = 'r1.21.0'
!python -m pip install "git+https://github.com/NVIDIA/NeMo.git@${BRANCH}#egg=nemo_toolkit[all]"
```
It will get below errors due to `${BRANCH}`. Removing the curly braces fixes the error.
```shell
DEPRECATION: git+https://github.com/NVIDIA/NeMo.git@.21.0#egg=nemo_toolkit[all] contains an egg fragment with a non-PEP 508 name pip 25.0 will enforce this behaviour change. A possible replacement is to use the req @ url syntax, and remove the egg fragment. Discussion can be found at https://github.com/pypa/pip/issues/11617
Collecting nemo_toolkit[all]
  Cloning https://github.com/NVIDIA/NeMo.git (to revision .21.0) to /tmp/pip-install-gzajrtok/nemo-toolkit_d9d14f8e02594d728021197cbfbd43ee
  Running command git clone --filter=blob:none --quiet https://github.com/NVIDIA/NeMo.git /tmp/pip-install-gzajrtok/nemo-toolkit_d9d14f8e02594d728021197cbfbd43ee
  WARNING: Did not find branch or tag '.21.0', assuming revision or ref.
  Running command git checkout -q .21.0
  error: pathspec '.21.0' did not match any file(s) known to git
  error: subprocess-exited-with-error
  
  × git checkout -q .21.0 did not run successfully.
  │ exit code: 1
  ╰─> See above for output.
  
  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× git checkout -q .21.0 did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.
\nRemember to restart the runtime for the kernel to pick up any upgraded packages (e.g. matplotlib)!\nAlternatively, you can uncomment the exit() below to crash and restart the kernel, in the case\nthat you want to use the "Run All Cells" (or similar) option.\n
```

**Change 2**
remove pynini installation since it is now a dependency of nemo-text-processing lib.

**Collection**: [Note which collection this PR will affect]

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
```

# Before your PR is "Ready for review"
**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?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
