minor #5338 Text in index.html.twig for The Big Picture wrong (BT643)

This PR was merged into the 2.3 branch.

Discussion
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Text in index.html.twig for The Big Picture wrong

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all

The text in the standard "index.html.twig" on "The Big Picture" documentation for the "Templates" section is wrong and needs updating.

The documentation shows: "<h1>Welcome to Symfony!</h1>" but the actual file (by default) has "Homepage.".

Nothing major but a bit misleading and confusing for a brand new user.

Commits
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bdada47 Text in index.html.twig for The Big Picture wrong

Author: wouterj

quick_tour/the_big_picture.rst

@@ -254,7 +254,7 @@ you'll see the following code:
     {% extends 'base.html.twig' %}
 
     {% block body %}
-        <h1>Welcome to Symfony!</h1>
+        Homepage.
     {% endblock %}
 
 This template is created with `Twig`_, a new template engine created for