[PHP-Symfony] fix name mismatch in generated README (#15961) (#15963)

This fixes the generated README so it  suggests to implement the `class DefaultApi`
in a file named DefaultApi.php (instead of DefaultApiInterface.php)

Author: gturri

modules/openapi-generator/src/main/resources/php-symfony/README.mustache

@@ -78,7 +78,7 @@ Step 4: Implement the API calls:
 
 ```php
 <?php{{#apiInfo}}{{#apis}}{{#-first}}{{#operations}}
-// src/Acme/MyBundle/Api/{{classname}}.php
+// src/Acme/MyBundle/Api/{{baseName}}Api.php
 
 namespace Acme\MyBundle\Api;
 

samples/server/petstore/php-symfony/SymfonyBundle-php/README.md

@@ -67,7 +67,7 @@ Step 4: Implement the API calls:
 
 ```php
 <?php
-// src/Acme/MyBundle/Api/PetApiInterface.php
+// src/Acme/MyBundle/Api/PetApi.php
 
 namespace Acme\MyBundle\Api;