Update egui_commonmark (rerun-io#5864)

### What
Seems pretty safe - i tested a handful of examples, and all their inline
markdown looks as good or better.

https://github.com/lampsitter/egui_commonmark/blob/master/CHANGELOG.md

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[rerun.io/viewer](https://rerun.io/viewer/pr/5864)
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5864?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5864?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5864)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

Cargo.toml

@@ -94,7 +94,7 @@ egui = { version = "0.27.2", features = [
   "puffin",
   "rayon",
 ] }
-egui_commonmark = { version = "0.14", default-features = false }
+egui_commonmark = { version = "0.15", default-features = false }
 egui_extras = { version = "0.27.2", features = ["http", "image", "puffin"] }
 egui_plot = "0.27.2"
 egui_tiles = "0.8.0"