docs: fix code example typo in reactivity.md (#1230)

Author: Seemspyo

docs/framework/react/guides/reactivity.md

@@ -31,12 +31,12 @@ While it IS possible to omit the selector, resist the urge as omitting it would
 The `form.Subscribe` component is best suited when you need to react to something within the UI of your component. For example, showing or hiding ui based on the value of a form field.
 
 ```tsx
-<form.Subscribe>
+<form.Subscribe
   selector={(state) => state.firstName}
   children={(firstName) => (
     <form.Field>{(field) => <input name="lastName" value={field.state.lastName} onChange={field.handleChange}/></form.Field>
   )}
-</form.Subscribe>
+/>
 ```
 
 > The `form.Subscribe` component doesn't trigger component-level re-renders. Anytime the value subscribed to changes, only the `form.Subscribe` component re-renders.