Fix prototype of astFromValue (graphql#643)

There is a mismatch between what is stated in the docs and the typescript definition. From typescript definition:
```
export function astFromValue(value: any, type: GraphQLInputType): Maybe<ValueNode>;
```

Author: livingmine

site/graphql-js/APIReference-Utilities.md

@@ -189,7 +189,7 @@ corresponding GraphQLType from that schema.
 ```js
 function astFromValue(
   value: any,
-  type?: ?GraphQLType
+  type: GraphQLInputType
 ): ?Value
 ```
 Produces a GraphQL Input Value AST given a JavaScript value.