[docs] remove duplicate 'available'

doc/testing.rst

@@ -55,7 +55,7 @@ page loaded correctly and contains our form.
         }
 
 Here you see some of the available assertions. There is a full list available
-available in the `Writing Tests for PHPUnit
+in the `Writing Tests for PHPUnit
 <http://www.phpunit.de/manual/current/en/writing-tests-for-phpunit.html>`_
 section of the PHPUnit documentation.