minor #16474 [Testing] Removed redundancy in package requirements (fridde)

This PR was merged into the 6.0 branch.

Discussion
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[Testing] Removed redundancy in package requirements

Acoording to [symfony/testpack](https://github.com/symfony/test-pack/blob/main/composer.json) the ```phpunit/phpunit``` package is explictly included.

There might be a point in showing both for clarity, but for me this redundancy created more confusion than it had explanation value.

I do understand that this might be a slightly controversial PR.

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Commits
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6667e41 Removed redundancy in package requirements

Author: javiereguiluz

testing.rst

@@ -15,13 +15,13 @@ Symfony integrates with an independent library called `PHPUnit`_ to give
 you a rich testing framework. This article won't cover PHPUnit itself,
 which has its own excellent `documentation`_.
 
-Before creating your first test, install ``phpunit/phpunit`` and the
-``symfony/test-pack``, which installs some other packages providing useful
+Before creating your first test, install ``phpunit/phpunit`` by installing
+``symfony/test-pack``, which also installs some other packages providing useful
 Symfony test utilities:
 
 .. code-block:: terminal
 
-    $ composer require --dev phpunit/phpunit symfony/test-pack
+    $ composer require --dev symfony/test-pack
 
 After the library is installed, try running PHPUnit: