minor symfony#2234 [Turbo][Testing] Fix to the right test (GrinWay)

This PR was merged into the 2.x branch.

Discussion
----------

[Turbo][Testing] Fix to the right test

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Issues        | Fixing the test
| License       | MIT

In order to test the Turbo loading you don't have to use `assertSelectorTextContains` method, instead of it use `assertSelectorWillContain` cuz it's async javascript

Commits
-------

535439f [Turbo][Testing] Fix to the right test

Author: kbond

src/Turbo/doc/index.rst

@@ -276,7 +276,7 @@ Symfony.
             $client->request('GET', '/');
 
             $client->clickLink('This block is scoped, the rest of the page will not change if you click here!');
-            $this->assertSelectorTextContains('body', 'This will replace the content of the Turbo Frame!');
+            $this->assertSelectorWillContain('body', 'This will replace the content of the Turbo Frame!');
         }
     }