minor symfony#10671 Update custom_constraint.rst (thomasbisignani)

This PR was submitted for the 4.1 branch but it was merged into the 2.8 branch instead (closes symfony#10671).

Discussion
----------

Update custom_constraint.rst

Just added the constraint check to limit the `ContainsAlphanumericValidator` use with the `ContainsAlphanumeric` constraint.

Commits
-------

bb30126 Update custom_constraint.rst

Author: javiereguiluz

validation/custom_constraint.rst

@@ -65,6 +65,10 @@ The validator class is also simple, and only has one required method ``validate(
     {
         public function validate($value, Constraint $constraint)
         {
+            if (!$constraint instanceof ContainsAlphanumeric) {
+               throw new UnexpectedTypeException($constraint, ContainsAlphanumeric::class);
+            }
+            
             // custom constraints should ignore null and empty values to allow
             // other constraints (NotBlank, NotNull, etc.) take care of that
             if (null === $value || '' === $value) {