[Filesystem] [Serializer] fixes English grammar typo

Author: hhamon

Tests/FilesystemTest.php

@@ -801,7 +801,7 @@ public function testSymlink()
         $file = $this->workspace.\DIRECTORY_SEPARATOR.'file';
         $link = $this->workspace.\DIRECTORY_SEPARATOR.'link';
 
-        // $file does not exists right now: creating "broken" links is a wanted feature
+        // $file does not exist right now: creating "broken" links is a wanted feature
         $this->filesystem->symlink($file, $link);
 
         $this->assertTrue(is_link($link));