Removing the recommended Constraints subdirectory

Since both the Constraint and the Validator are in the same directory, I can't see any reason for putting them into a *sub*directory called `Constraints`. In other words: When keeping the `Constraints` subdirectory, what would be the recommended contents of the `Validator` directory?
* If you agree in principle, I'll change the other occurrences too.
* If you don't agree (i.e. want to keep the `Constraints` subdirectory), I suggest to rename it to `Constraint`, since all other directory names are singular too.

Author: ThomasLandauer

validation/custom_constraint.rst

@@ -14,8 +14,8 @@ Creating the Constraint Class
 
 First you need to create a Constraint class and extend :class:`Symfony\\Component\\Validator\\Constraint`::
 
-    // src/Validator/Constraints/ContainsAlphanumeric.php
-    namespace App\Validator\Constraints;
+    // src/Validator/ContainsAlphanumeric.php
+    namespace App\Validator;
 
     use Symfony\Component\Validator\Constraint;