Merge pull request #571 from cedricziel/patch-4

Correct sentence in validation.md

Author: dunglas

core/validation.md

@@ -7,7 +7,7 @@ if you want to.
 
 ## Validating Submitted Data
 
-Validating submitted data is simple as adding [Symfony's built-in constraints](http://symfony.com/doc/current/reference/constraints.html)
+Validating submitted data is as simple as adding [Symfony's built-in constraints](http://symfony.com/doc/current/reference/constraints.html)
 or [custom constraints](http://symfony.com/doc/current/validation/custom_constraint.html) directly in classes marked with
 the `@ApiResource` annotation: