minor #7082 Update metadata.rst (fberthereau)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #7082).

Discussion
----------

Update metadata.rst

True does not exist

Commits
-------

c15a5d1 Update metadata.rst

components/validator/metadata.rst

@@ -58,7 +58,7 @@ Then, add the Validator component configuration to the class::
     {
         public static function loadValidatorMetadata(ClassMetadata $metadata)
         {
-            $metadata->addGetterConstraint('passwordSafe', new Assert\True(array(
+            $metadata->addGetterConstraint('passwordSafe', new Assert\IsTrue(array(
                 'message' => 'The password cannot match your first name',
             )));
         }