Update page_creation.rst

[aliber4079]

making it friendlier to newbies

[javiereguiluz]

@aliber4079 thanks and congrats on your first Symfony Docs contribution 🎉

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A comment for @symfony/team-symfony-docs I know that our policy is to NOT include the <?php opening tag in docs unless they are aimed to absolute beginners. But we've been asked to add this specific opening tag a few times (here's the last time: #18408). So, I think it's fair to assume that we need that opening tag in this code example. Thanks!