docs: add break to switch block (#769)

I get that there's only one `case` here but for people plucking this example
and then using it to build out their own reducer, they'll immediately run into
issues as soon as they add a second `case` with no `break` in the first.

website/docs/example-reducer.mdx

@@ -67,6 +67,7 @@ const byId = produce((draft, action) => {
 			action.products.forEach(product => {
 				draft[product.id] = product
 			})
+			break
 	}
 }, INITIAL_STATE)
 ```