[doc] Updating the colab link

Author: blefaudeux

README.md

@@ -7,7 +7,7 @@
 [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](CONTRIBUTING.md)
 [![codecov](https://codecov.io/gh/facebookresearch/xformers/branch/main/graph/badge.svg?token=PKGKDR4JQM)](https://codecov.io/gh/facebookresearch/xformers)
 [![black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
-[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://github.com/facebookresearch/xformers/tree/colab_example/docs/source/xformers_mingpt.ipynb)
+[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/facebookresearch/xformers/blob/main/docs/source/xformers_mingpt.ipynb)
 
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