fix(docs): update shapes docs add the array of defined shapes (#1949)

### Change Type

- [x] `documentation` — Changes to the documentation only[^2]

### Release Notes

This pr add the custom defined shapes that's being passed to Tldraw

Co-authored-by: Steve Ruiz <steveruizok@gmail.com>

docs/docs/shapes.mdx

@@ -142,6 +142,8 @@ import '@tldraw/tldraw/tldraw.css'
 
 const MyCardShape = defineShape('card', { util: CardShapeUtil })
 
+const MyCustomShapes = [MyCardShape]
+
 export default function () {
 	return (
 		<div style={{ position: 'fixed', inset: 0 }}>