minor #3473 Fix length filter documentation about computation (VincentLanglet)
minor #3473 Fix length filter documentation about computation (VincentLanglet)

This PR was merged into the 1.x branch.

Discussion
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Fix length filter documentation about computation

According to https://github.com/twigphp/Twig/blob/1.x/src/Extension/CoreExtension.php#L1317

Commits
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473f73ef Fix length filter doc

twigphp/Twig,twigphp/Twig,twigphp/Twig