Fix use of removed method

`Validator::provider()` switched to `Validator::setProvider()`

Author: PhantomWatson

docs/validation.rst

@@ -20,11 +20,11 @@ Since by default, no validation rules are loaded, you should start with that:
 
     <?php
 
-        $validator->provider('upload', \Josegonzalez\Upload\Validation\UploadValidation::class);
+        $validator->setProvider('upload', \Josegonzalez\Upload\Validation\UploadValidation::class);
         // OR
-        $validator->provider('upload', \Josegonzalez\Upload\Validation\ImageValidation::class);
+        $validator->setProvider('upload', \Josegonzalez\Upload\Validation\ImageValidation::class);
         // OR
-        $validator->provider('upload', \Josegonzalez\Upload\Validation\DefaultValidation::class);
+        $validator->setProvider('upload', \Josegonzalez\Upload\Validation\DefaultValidation::class);
 
     ?>