salaryDataFetcher returns Mono

We need the return type to be a Reactor type for the context to be
propagated.

samples/employee-service/src/main/java/io/spring/sample/graphql/DataFetchers.java

@@ -22,7 +22,7 @@ public class DataFetchers {
 
     public DataFetcher salaryDataFetcher = env -> {
         Employee employee = env.getSource();
-        return salaryService.getSalaryForEmployee(employee).toFuture();
+        return salaryService.getSalaryForEmployee(employee);
     };
 
     public DataFetcher updateSalaryFetcher = env -> {