fix link for devmode to git repo

docs/source/developer/contributing.rst

@@ -99,7 +99,7 @@ Installing Node.js and jlpm
 Building JupyterLab from its GitHub source code requires Node.js. The
 development version requires Node.js version 10+, as defined in the
 ``engines`` specification in
-`dev_mode/package.json <dev_mode/package.json>`__.
+`dev_mode/package.json <https://github.com/jupyterlab/jupyterlab/blob/master/dev_mode/package.json>`__.
 
 If you use ``conda``, you can get it with: