annotate asset props, correct image/video confusion (#2399)

Annotates the asset props example

### 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

### Test Plan

1. Add a step-by-step description of how to test your PR here.
2.

- [ ] Unit Tests
- [ ] End to end tests

### Release Notes

- Annotates the asset props example and fixes a mistake in the existing
comments

apps/examples/src/examples/asset-props/AssetPropsExample.tsx

@@ -7,7 +7,7 @@ export default function AssetPropsExample() {
 			<Tldraw
 				// only allow jpegs
 				acceptedImageMimeTypes={['image/jpeg']}
-				// don't allow any images
+				// don't allow any videos
 				acceptedVideoMimeTypes={[]}
 				// accept images of any dimension
 				maxImageDimension={Infinity}
@@ -17,3 +17,9 @@ export default function AssetPropsExample() {
 		</div>
 	)
 }
+
+/* 
+This example shows how to use props on the Tldraw component to control what types of
+assets can be uploaded.
+
+*/