Replace removed rule from documentation

The "Age" was removed but the documentation was not completely updated.

docs/feature-guide.md

@@ -42,7 +42,7 @@ Is possible to validate its attributes in a single chain:
 
 ```php
 $userValidator = v::attribute('name', v::stringType()->length(1, 32))
-                  ->attribute('birthdate', v::date()->age(18));
+                  ->attribute('birthdate', v::date()->minAge(18));
 
 $userValidator->validate($user); // true
 ```