Provide realistic code example

Current code example does `afDb.object('items').snapshotChanges()` which would give a key value of 'items' followed by all items.

Author: bob-lee

docs/version-5-upgrade.md

@@ -41,11 +41,14 @@ constructor(afDb: AngularFireDatabase) {
 ### 5.0
 ```ts
 constructor(afDb: AngularFireDatabase) {
-  afDb.object('items').snapshotChanges().map(action => {
-    const $key = action.payload.key;
-    const data = { $key, ...action.payload.val() };
-    return data;
-  }).subscribe(item => console.log(item.$key));
+  afDb.list('items').snapshotChanges().map(action => {
+    const arr = [];
+    action.forEach(e => {
+      const $key = e.key;
+      arr.push({ $key, ...e.payload.val() });
+    });
+    return arr;
+  }).subscribe(items => items.forEach(item => console.log(item.$key)));
 }
 ```