docs: Field.md: Sync validate and warning signature (#4300)
docs: Field.md: Sync validate and warning signature (#4300)

Updated the signature of validate and warning props as we can see it in the example section and in the propTypes of Field :

```js
 // ...
    validate: PropTypes.oneOfType([
      PropTypes.func,
      PropTypes.arrayOf(PropTypes.func)
    ]),
    warn: PropTypes.oneOfType([
      PropTypes.func,
      PropTypes.arrayOf(PropTypes.func)
    ])
// ...
```
erikras/redux-form,erikras/redux-form