Fix typo

Summary: Pull Request resolved: facebookresearch#171

Differential Revision: D18137764

Pulled By: ppwwyyxx

fbshipit-source-id: 4e41b04db86b18a208603a76ee4213ed227ad25f

Author: jaesuny

docs/tutorials/extend.md

@@ -31,7 +31,7 @@ In detectron2, there are two types of interfaces that address this tension toget
    included in detectron2, these well-defined components can be reused.
 
 
-If you only need the standard behavior, the [Beginner's Tutorial](getting_started)
+If you only need the standard behavior, the [Beginner's Tutorial](getting_started.html)
 should suffice. If you need to extend detectron2 to your own needs,
 see the following tutorials for more details: