remove the Slug constraint

Author: xabbuh

reference/constraints/Slug.rst

@@ -33,7 +33,6 @@ String Constraints
 * :doc:`NotCompromisedPassword </reference/constraints/NotCompromisedPassword>`
 * :doc:`PasswordStrength </reference/constraints/PasswordStrength>`
 * :doc:`Regex </reference/constraints/Regex>`
-* :doc:`Slug </reference/constraints/Slug>`
 * :doc:`Twig </reference/constraints/Twig>`
 * :doc:`Ulid </reference/constraints/Ulid>`
 * :doc:`Url </reference/constraints/Url>`