Update the link for the MATLAB `.gitignore` file

We say: A .gitignore file for a typical MATLAB toolbox project can be found [here](https://github.com/mathworks/gitignore/blob/main/Global/MATLAB.gitignore).

GitHub finally accepted our pull request, so we can point to the official .gitignore [here](https://github.com/github/gitignore/blob/main/Global/MATLAB.gitignore).