minor #14526 [Mailer] Update mailer.rst (KamilPesek)

This PR was submitted for the 5.1 branch but it was merged into the 4.4 branch instead.

Discussion
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[Mailer] Update mailer.rst

The registeted namespace is "styles", not "css".

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Commits
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beea71c Update mailer.rst

Author: OskarStark

mailer.rst

@@ -586,7 +586,7 @@ arguments to the filter:
 
 .. code-block:: html+twig
 
-    {% apply inline_css(source('@css/email.css')) %}
+    {% apply inline_css(source('@styles/email.css')) %}
         <h1>Welcome {{ username }}!</h1>
         {# ... #}
     {% endapply %}