Fix broken autoloading due to composer.json discrepancy

[SeinopSys]

Due to the difference between the autoload namespace specified in composer.json and the actual namespace in the file that contains the class this repository currently breaks Composer commands after adding it to the $listen[] array of app/Providers/EventServiceProvider.php in a Laravel project.

$ composer update
Loading composer repositories with package information
Nothing to install or update
Generating optimized autoload files
> Illuminate\Foundation\ComposerScripts::postAutoloadDump
> @php artisan package:discover

                                                                               
  [ReflectionException]                                                        
  Class \SocialiteProviders\Deviantart\DeviantartExtendSocialite does not exi  
  st                                                                           
                                                                               

Script @php artisan package:discover handling the post-autoload-dump event returned with error code 1

I changed the namespace in the php file instead of copying the one in the file to the composer.json because this page suggests the use of that form over the one found in the file.

While this should work for now, I would highly suggest that eventually both this repository and the providers site be updated with the new stylization of the name: DeviantArt.

[SeinopSys]

@faustbrian Thank you. I'd suggest updating the Packagist cache of the repository at your earliest convenience to allow for ^3.0.1 to be specified as a Composer dependency.