update PR template

Author: ytdHuang

.github/pull_request_template.md

@@ -2,6 +2,7 @@
 Thank you for contributing to Tutorials for Quantum Toolbox in `Julia`! Please make sure you have finished the following tasks before opening the PR.
 
 - [ ] Please read [Contributing to QuantumToolbox.jl](https://qutip.org/QuantumToolbox.jl/stable/resources/contributing).
+- [ ] The (last update) `date` were modified for new or updated tutorials.
 - [ ] All tutorials were able to render locally by running: `make render`.
 
 Request for a review after you have completed all the tasks. If you have not finished them all, you can also open a [Draft Pull Request](https://github.blog/2019-02-14-introducing-draft-pull-requests/) to let the others know this on-going work.