Updated colab links in readme of examples
# What does this PR do?
Updated the Google Colab links for the The Big Table of Tasks in the examples folder readme.
Google Colab links were replace to the appropriate examples found [here](https://github.com/huggingface/notebooks/tree/master/examples)

text_generation.ipynb is not present in the [notebook repo](https://github.com/huggingface/notebooks/tree/master/examples). Will text_generation.ipynb be added in the future?


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?
@sgugger

