[fix] broken link in README from #314

The format changed in 1fcf066, but the link wasn't updated -- until now.

README.rst

 Runs robot (Selenium) tests, you must have ``firefox`` installed because
 this functional tests actually run the browser and perform operations on
 it. Also searx is executed with
-`settings\_robot <https://github.com/asciimoo/searx/blob/master/searx/settings_robot.py>`__.
+`settings\_robot <https://github.com/asciimoo/searx/blob/master/searx/settings_robot.yml>`__.
 
 ``make flake8``
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