update link to docs for watch() api in codesandbox with watch() API examples at FAQ section "watch vs getValues vs state"

on https://react-hook-form.com/faqs#watchvsgetValuesvsstate there is link to codesandbox that includes old link to page that doesnt exist:
<a href="https://react-hook-form.com/api#watch">https://react-hook-form.com/api#watch</a>

need to change link from https://react-hook-form.com/api#watch to actual link https://react-hook-form.com/docs/useform/watch