Correct type of ref in forwardRef render() (#13100)

`React$ElementRef<T>` is the type of the ref _instance_ for a component of type T, whereas `React$Ref<T>` is the type of the ref _prop_ for a component of type T, which seems to be the intended type here.

packages/react/src/forwardRef.js

@@ -10,7 +10,7 @@ import {REACT_FORWARD_REF_TYPE} from 'shared/ReactSymbols';
 import warning from 'shared/warning';
 
 export default function forwardRef<Props, ElementType: React$ElementType>(
- render: (props: Props, ref: React$ElementRef<ElementType>) => React$Node,
+ render: (props: Props, ref: React$Ref<ElementType>) => React$Node,
 ) {
  if (__DEV__) {
  if (typeof render !== 'function') {