docs: fix ordering on null data source examples (#1483)

Author: ralacerda

Diff for: docs/guide/realtime-data.md

@@ -101,21 +101,21 @@ On top of that, VueFire also allows `null` as a value for the data source. This
 
 ```ts
 const user = useCurrentUser()
-const myContactList = useCollection(() =>
+const myContactList = useDatabaseList(() =>
   user.value
-    ? // Firebase will error if a null value is passed to `collection()`
-      collection(db, 'users', user.value.id, 'contacts')
+    ? // Firebase will error if a null value is passed to `dbRef()`
+      dbRef(db, 'users', user.value.id, 'contacts')
     : // this will be considered as no data source
       null
 )
 ```
 
 ```ts
 const user = useCurrentUser()
-const myContactList = useDatabaseList(() =>
+const myContactList = useCollection(() =>
   user.value
-    ? // Firebase will error if a null value is passed to `dbRef()`
-      dbRef(db, 'users', user.value.id, 'contacts')
+    ? // Firebase will error if a null value is passed to `collection()`
+      collection(db, 'users', user.value.id, 'contacts')
     : // this will be considered as no data source
       null
 )