docs(rtdb): edit doc for sample code may cause confusion (#1260)

Author: adnan333bd

docs/version-5-upgrade.md

@@ -41,7 +41,7 @@ constructor(afDb: AngularFireDatabase) {
 ### 5.0
 ```ts
 constructor(afDb: AngularFireDatabase) {
-  afDb.object('items').snapshotChanges().map(action => {
+  afDb.object('items/1').snapshotChanges().map(action => {
     const $key = action.payload.key;
     const data = { $key, ...action.payload.val() };
     return data;