fix missing reference error of $books variable

Author: joonlabs

examples/books/__schema.php

@@ -174,7 +174,7 @@ function ($_, $args) use ($books) {
 /**
  * Create the Mutation-Type
  */
-$mutationType = new GraphQLObjectType("Mutation", "Root Mutation", function () use (&$bookType) {
+$mutationType = new GraphQLObjectType("Mutation", "Root Mutation", function () use (&$bookType, &$books) {
     return [
         new GraphQLTypeField(
             "incrementBookIds",