fix typo

Author: xabbuh

reference/configuration/kernel.rst

@@ -288,7 +288,7 @@ have deleted it entirely (for example in the production servers), override the
 
         public function getProjectDir(): string
         {
-            // when defining a hardcoded string, don't add the triailing slash to the path
+            // when defining a hardcoded string, don't add the trailing slash to the path
             // e.g. '/home/user/my_project', '/app', '/var/www/example.com'
             return \dirname(__DIR__);
         }