Fix Flow typing for Flow v0.82

https://github.com/facebook/flow/releases/tag/v0.82.0

“Removed the ability to use functions as type annotations.”

src/preact.js.flow

@@ -2,7 +2,7 @@
 
 import { createElement, cloneElement, Component, type Node } from 'react';
 
-declare var h: createElement;
+declare var h: typeof createElement;
 
 declare function render(vnode: Node, parent: Element, toReplace?: Element): Element;