Adjust code sample to use new offset syntax (#3258)

Co-authored-by: sambler <[email protected]>

getting_started/step_by_step/your_first_game.rst

@@ -815,7 +815,7 @@ Note that a new instance must be added to the scene using
 
     func _on_MobTimer_timeout():
         # Choose a random location on Path2D.
-        $MobPath/MobSpawnLocation.set_offset(randi())
+        $MobPath/MobSpawnLocation.offset = randi()
         # Create a Mob instance and add it to the scene.
         var mob = Mob.instance()
         add_child(mob)