Docs Improvement - In ZSH, not using ' ' around pip install fails, fix it (#22045)

In ZSH, not using ' ' around pip install fails

Running 
```
pip install transformers[torch]
```
in the default ZSH terminal will fail with the error `zsh: no matches found: transformers[torch]`

The solution is to wrap the installation path in ' ' like 
```
pip install 'transformers[torch]'
```

Relevant StackOverflow: https://stackoverflow.com/questions/30539798/zsh-no-matches-found-requestssecurity

docs/source/en/installation.mdx

@@ -54,19 +54,19 @@ pip install transformers
 For CPU-support only, you can conveniently install 🤗 Transformers and a deep learning library in one line. For example, install 🤗 Transformers and PyTorch with:
 
 ```bash
-pip install transformers[torch]
+pip install 'transformers[torch]'
 ```
 
 🤗 Transformers and TensorFlow 2.0:
 
 ```bash
-pip install transformers[tf-cpu]
+pip install 'transformers[tf-cpu]'
 ```
 
 🤗 Transformers and Flax:
 
 ```bash
-pip install transformers[flax]
+pip install 'transformers[flax]'
 ```
 
 Finally, check if 🤗 Transformers has been properly installed by running the following command. It will download a pretrained model: