Working binder link

docs/getting_started/online_sandbox/sandbox_intro.md

@@ -12,6 +12,6 @@ You can open the sandbox tutorial in one of two ways. You may be forced to use o
 
 - Open in [Google Colab](https://githubtocolab.com/casact/chainladder-python/blob/master/docs/getting_started/online_sandbox/sandbox_workbook_blank.ipynb) (faster, but a free Google Account is required)
 
-- Open in [Binder](https://mybinder.org/v2/gh/casact/chainladder-python/blob/master/docs/getting_started/online_sandbox/sandbox_workbook_blank.ipynb) (slower, no sign up required)
+- Open in [Binder](https://mybinder.org/v2/gh/casact/chainladder-python/master?urlpath=tree/docs/getting_started/online_sandbox/sandbox_workbook_blank.ipynb) (slower, no sign up required)
 
 Just in case you want a copy of the sandbox tutorial notebook, you can download it [here](https://github.com/casact/chainladder-python/blob/master/docs/getting_started/online_sandbox/sandbox_workbook_blank.ipynb).