DOC change doc link about testing from stable (nose) to dev (pytest)

We have changed to pytest but the stable doc is about nose.
Already gain approval from @lesteve in scikit-learn#9840.
Nothing to check so I push directly.

Author: qinhanmin2014

README.rst

@@ -114,7 +114,7 @@ source directory (you will need to have the ``pytest`` package installed)::
 
     pytest sklearn
 
-See the web page http://scikit-learn.org/stable/developers/advanced_installation.html#testing
+See the web page http://scikit-learn.org/dev/developers/advanced_installation.html#testing
 for more information.
 
     Random number generation can be controlled during testing by setting