minor #15782 [Mercure] Remove wrong parameter in MockHub (elliotbruneel)

This PR was submitted for the 5.3 branch but it was merged into the 4.4 branch instead.

Discussion
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[Mercure] Remove wrong parameter in MockHub

There is no parameter in `MockHub` to specify the hub name (`default`)

`MockHub` constructor : https://github.com/symfony/mercure/blob/main/src/MockHub.php#L30-L35

```php
    /**
     * `@param` (callable(Update): string) $publisher
     */
    public function __construct(
        string $url,
        TokenProviderInterface $jwtProvider,
        callable $publisher,
        TokenFactoryInterface $jwtFactory = null,
        string $publicUrl = null
    )
```

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Commits
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28cbc5d Remove wrong parameter in MockHub

Author: javiereguiluz

mercure.rst

@@ -576,7 +576,7 @@ You can instead make use of the `MockHub`::
     {
         public function testPublishing()
         {
-            $hub = new MockHub('default', 'https://internal/.well-known/mercure', new StaticTokenProvider('foo'), function(Update $update): string {
+            $hub = new MockHub('https://internal/.well-known/mercure', new StaticTokenProvider('foo'), function(Update $update): string {
                 // $this->assertTrue($update->isPrivate());
 
                 return 'id';