[ExpressionLanguage] Readd a needed reference

Author: javiereguiluz

components/expression_language/syntax.rst

@@ -370,3 +370,5 @@ expressions (e.g. the request, the current user, etc.):
 * :doc:`Variables available in security expressions </security/expressions>`;
 * :doc:`Variables available in service container expressions </service_container/expression_language>`;
 * :ref:`Variables available in routing expressions <routing-matching-expressions>`.
+
+.. _`null-coalescing operator`: https://www.php.net/manual/en/language.operators.comparison.php#language.operators.comparison.coalesce