Update type GraphQLResolveInfo description (graphql#312)

In source code `fieldASTs` was renamed to `fieldASTs `

Author: karudo

site/graphql-js/APIReference-TypeSystem.md

@@ -264,7 +264,7 @@ type GraphQLFieldResolveFn = (
 
 type GraphQLResolveInfo = {
   fieldName: string,
-  fieldASTs: Array<Field>,
+  fieldNodes: Array<Field>,
   returnType: GraphQLOutputType,
   parentType: GraphQLCompositeType,
   schema: GraphQLSchema,