Fix outdated link in the docs (#7971)

Fixes #7968

### Description

Fix outdated link in the docs

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: YunLiu <[email protected]>

docs/source/index.rst

@@ -37,7 +37,7 @@ Features
 Getting started
 ---------------
 
-`MedNIST demo <https://colab.research.google.com/drive/1wy8XUSnNWlhDNazFdvGBHLfdkGvOHBKe>`_ and `MONAI for PyTorch Users <https://colab.research.google.com/drive/1boqy7ENpKrqaJoxFlbHIBnIODAs1Ih1T>`_ are available on Colab.
+`MedNIST demo <https://colab.research.google.com/github/Project-MONAI/tutorials/blob/main/2d_classification/mednist_tutorial.ipynb>`_ and `MONAI for PyTorch Users <https://colab.research.google.com/github/Project-MONAI/tutorials/blob/main/modules/developer_guide.ipynb>`_ are available on Colab.
 
 Examples and notebook tutorials are located at `Project-MONAI/tutorials <https://github.com/Project-MONAI/tutorials>`_.