Bump @microsoft/sp-core-library from 1.15.2 to 1.16.0 in /graph-tutorial

Bumps [@microsoft/sp-core-library](http://aka.ms/spfx) from 1.15.2 to 1.16.0.

---
updated-dependencies:
- dependency-name: "@microsoft/sp-core-library"
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]