Update flow definitions for next

Author: captainsafia

flow-typed/npm/next_v7.x.x.js

@@ -153,6 +153,7 @@ declare module "next/document" {
 
   declare export var Head: Class<React$Component<any, any>>;
   declare export var Main: Class<React$Component<any, any>>;
+  declare export var Html: Class<React$Component<any, any>>;
   declare export var NextScript: Class<React$Component<any, any>>;
   declare export default Class<React$Component<any, any>> & {
     getInitialProps: (ctx: Context) => Promise<*>,