Fix warnings in reference docs code snippets

spring-graphql-docs/build.gradle

@@ -24,6 +24,7 @@ dependencies {
 	api 'org.springframework:spring-websocket'
 	api 'org.springframework:spring-messaging'
 	api 'org.springframework.data:spring-data-commons'
+	api 'com.querydsl:querydsl-core'
 	asciidoctorExtensions 'io.spring.asciidoctor.backends:spring-asciidoctor-backends:0.0.3'
 }
 

spring-graphql-docs/src/main/java/org/springframework/graphql/docs/graalvm/server/GraphQlConfiguration.java

@@ -30,7 +30,7 @@ public class GraphQlConfiguration {
 
 	@Bean
 	RuntimeWiringConfigurer customWiringConfigurer(BookRepository bookRepository) { // <1>
-		DataFetcher dataFetcher = QuerydslDataFetcher.builder(bookRepository).single();
+		DataFetcher<Book> dataFetcher = QuerydslDataFetcher.builder(bookRepository).single();
 		return wiringBuilder -> wiringBuilder
 				.type("Query", builder -> builder.dataFetcher("book", dataFetcher)); // <2>
 	}