minor symfony#9556 Fix little typo (mercuryseries)

This PR was merged into the 4.0 branch.

Discussion
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Fix little typo

Should be head_js or better to remove it like in the head_css example above.

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Commits
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487cae4 Fix little typo

Author: javiereguiluz

form/bootstrap4.rst

@@ -16,7 +16,7 @@ template which other templates extend from):
     {% endblock %}
     {% block head_js %}
         <!-- Copy JavaScript from https://getbootstrap.com/docs/4.0/getting-started/introduction/#js -->
-    {% endblock head %}
+    {% endblock %}
 
 If your application uses modern front-end practices, it's better to use
 :doc:`Webpack Encore </frontend>` and follow :doc:`this tutorial </frontend/encore/bootstrap>`