Update the External content sources example. The comment was misleading
as it mentioned dragging a file, but the example only works when you
paste the `text/html` content.
https://github.com/tldraw/tldraw/assets/2523721/7c4193ff-e2ba-417f-95c0-7da1f10b585b
Improves https://github.com/tldraw/tldraw/issues/2200
### Change Type
- [ ] `patch` — Bug fix
- [ ] `minor` — New feature
- [ ] `major` — Breaking change
- [ ] `dependencies` — Changes to package dependencies[^1]
- [x] `documentation` — Changes to the documentation only[^2]
- [ ] `tests` — Changes to any test code only[^2]
- [ ] `internal` — Any other changes that don't affect the published
package[^2]
- [ ] I don't know
[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version
### Release Notes
- Improve the comment for one of our examples.
6f59e54da6