bug symfony#4984 Fix the example using SerializedParsedExpression (stof)

This PR was merged into the 2.5 branch.

Discussion
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Fix the example using SerializedParsedExpression

Closes symfony#4970

Commits
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b51bdbe Fix the example using SerializedParsedExpression

Author: wouterj

components/expression_language/caching.rst

@@ -64,6 +64,7 @@ Both ``evaluate()`` and ``compile()`` can handle ``ParsedExpression`` and
     // ...
 
     $expression = new SerializedParsedExpression(
+        '1 + 4',
         serialize($language->parse('1 + 4', array()))
     );