minor symfony#2145 [Icons] Fix issues found by DOCtor-RST (Kocal)

This PR was merged into the 2.x branch.

Discussion
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[Icons] Fix issues found by DOCtor-RST

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Issues        | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

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Found in https://github.com/symfony/ux/pull/2144/files.

Commits
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ba00d69 [Icons] Fix issues found by DOCtor-RST

Author: Kocal

src/Icons/doc/index.rst

@@ -352,7 +352,7 @@ creating shortcuts to icons you frequently use in your templates:
 
 Now, you can use the ``dots`` alias in your templates:
 
-.. code-block:: twig
+.. code-block:: html+twig
 
     {{ ux_icon('dots') }}
     {# with the previous configuration, this is the same as: #}