Fix error output of the psr-4 trailing backslash validation

src/Composer/Package/Loader/ValidatingArrayLoader.php

@@ -209,7 +209,7 @@ class ValidatingArrayLoader implements LoaderInterface
                 if ($type === 'psr-4') {
                     foreach ($typeConfig as $namespace => $dirs) {
                         if ($namespace !== '' && '\\' !== substr($namespace, -1)) {
-                            $this->errors[] = 'autoload.psr-4 : invalid value ('.$namespace.'), namespaces must end with a namespace separator, should be '.$namespace.'\\';
+                            $this->errors[] = 'autoload.psr-4 : invalid value ('.$namespace.'), namespaces must end with a namespace separator, should be '.$namespace.'\\\\';
                         }
                     }
                 }