fix missing change of singletonstructure in readme

Author: johguentner

README.md

@@ -51,7 +51,7 @@ $notion->databases()->find($databaseId);
 #### Fetch Page Information
 ```php
 // Returns a specific page
-$notion->pages()->find($yourPageId);
+\Notion::pages()->find($yourPageId);
 ```
 
 #### Search