Update demo deps to mitigate CVE-2021-38190

[urschrei]

• [x] I agree to follow the project's code of conduct.

• [ ] I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

Fixes https://github.com/georust/rstar/security/dependabot/1

[urschrei]

bors r=michaelkirk

[bors[bot]]

Build succeeded:

• ci result