Merge pull request #17 from andrew-demb/patch-1

Fix example snippet indentation

docs/book/diagnostics.md

@@ -47,7 +47,7 @@ $checkDbFile = new Callback(function() {
         return new Success('Db file is ok');
     }
 
-		return new Failure('There is something wrong with the db file');
+    return new Failure('There is something wrong with the db file');
 });
 ```