Fix reference object parameter

Author: Caitlin Bales (MSFT)

src/main/java/com/microsoft/graph/models/generated/BasePlannerExternalReferences.java

@@ -28,7 +28,7 @@
 /**
  * The class for the Base Planner External References.
  */
-public class BasePlannerExternalReferences extends HashMap<String, String> implements IJsonBackedObject {
+public class BasePlannerExternalReferences extends HashMap<String, Object> implements IJsonBackedObject {
 
     @SerializedName("@odata.type")
     @Expose(serialize = false)