minor #1021 Update testing.rst (1emming)

This PR was submitted for the master branch but it was merged into the 1.2 branch instead (closes #1021).

Discussion
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Update testing.rst

Fix dead link, @see:
https://github.com/silexphp/Silex/issues/1020

Commits
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8c8b6a67 Update testing.rst

doc/testing.rst

@@ -54,7 +54,7 @@ page loaded correctly and contains our form::
 
 Here you see some of the available assertions. There is a full list available
 in the `Writing Tests for PHPUnit
-<http://www.phpunit.de/manual/current/en/writing-tests-for-phpunit.html>`_
+<https://phpunit.de/manual/current/en/writing-tests-for-phpunit.html>`_
 section of the PHPUnit documentation.
 
 WebTestCase