minor #14046 [Notifier] Update SlackOptions code example (loru88)

This PR was merged into the 5.1 branch.

Discussion
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[Notifier] Update SlackOptions code example

If you copy and paste the SlackOptions as it was, Slack gives back the error `invalid blocks`.

When I was trying to use the Slack notifier component, I couldn't understand why and I found out is because Slack needs a working URL to an image, so I suggest to this change to help other developers when they will reach the same issue.

Apparently Slack does not support SVG and the logo from here https://symfony.com/logo are cropped in the Slack message. For this reason, I took the apple-touch icon URL, because it fits well in the Slack message.

Commits
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d6f8fd8 Update chatters.rst

Author: javiereguiluz

notifier/chatters.rst

@@ -77,7 +77,7 @@ some interactive options called `Block elements`_::
             ->text('The Symfony Community')
             ->accessory(
                 new SlackImageBlockElement(
-                    'https://example.com/symfony-logo.png',
+                    'https://symfony.com/favicons/apple-touch-icon.png',
                     'Symfony'
                 )
             )