Fix assert statement

[nacholibre]

In my local version of phpunit 9.5.9 $this->assertContains has the following signature:

public static function assertContains($needle, iterable $haystack, string $message = ''): void
{
    //..
}

Where $commandTester->getDisplay(); returns string and the test fails with

TypeError: Argument 2 passed to PHPUnit\Framework\Assert::assertContains() must be iterable, string given, called in..

[javiereguiluz]

Good catch Yoan! Thanks and congrats on your first Symfony Docs contribution 🎉