Fix quoting style consistency.

Author: ogizanagi

Validator/Constraints/UniqueEntityValidator.php

@@ -77,7 +77,7 @@ public function validate($entity, Constraint $constraint)
         $criteria = array();
         foreach ($fields as $fieldName) {
             if (!$class->hasField($fieldName) && !$class->hasAssociation($fieldName)) {
-                throw new ConstraintDefinitionException(sprintf("The field '%s' is not mapped by Doctrine, so it cannot be validated for uniqueness.", $fieldName));
+                throw new ConstraintDefinitionException(sprintf('The field "%s" is not mapped by Doctrine, so it cannot be validated for uniqueness.', $fieldName));
             }
 
             $criteria[$fieldName] = $class->reflFields[$fieldName]->getValue($entity);