minor symfony#10672 Documentation: copy-files.rst: fix missing code-block (chteuchteu)

This PR was submitted for the 4.1 branch but it was squashed and merged into the 3.4 branch instead (closes symfony#10672).

Discussion
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Documentation: copy-files.rst: fix missing code-block

The latest code block in [Copying & Referencing Images](https://symfony.com/doc/current/frontend/encore/copy-files.html) is missing a `code-block`

Commits
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a98e0d8 Documentation: copy-files.rst: fix missing code-block

Author: javiereguiluz

frontend/encore/copy-files.rst

@@ -40,7 +40,7 @@ files into your final output directory.
 
     +     .copyFiles({
     +         from: './assets/images',
-    + 
+    +
     +         // optional target path, relative to the output dir
     +         //to: 'images/[path][name].[ext]',
     +
@@ -54,6 +54,8 @@ include a hash based on their content.
 
 To render inside Twig, use the ``asset()`` function:
 
+.. code-block:: html+twig
+
     {# assets/images/logo.png was copied to web/build/logo.png #}
     <img src="{{ asset('build/logo.png') }}"