fixed StringExpressionLanguageProvider code example #4636
bug #4637 fixed StringExpressionLanguageProvider code example #4636 (danieleorler)

This PR was squashed before being merged into the 2.6 branch (closes #4637).

Discussion
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fixed StringExpressionLanguageProvider code example #4636

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6
| Fixed tickets | #4636

Commits
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33e0a7f fixed StringExpressionLanguageProvider code example #4636