minor symfony#6264 Update email.rst (mikaelz)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes symfony#6264).

Discussion
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Update email.rst

Use `name` defined in the controller.

Commits
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847db7e Update email.rst

Author: wouterj

cookbook/email/email.rst

@@ -131,6 +131,8 @@ template might look something like this:
 .. code-block:: html+jinja
 
     {# app/Resources/views/Emails/registration.html.twig #}
+    Hi {{ name }}
+    
     <h3>You did it! You registered!</h3>
 
     {# example, assuming you have a route named "login" #}