fix psalm issue

Author: Nyholm

test/unit/Formatter/JsonFormatterTest.php

@@ -76,7 +76,7 @@ public function testWrite(): void
         self::assertIsArray($data);
         self::assertEquals($expected, $data);
 
-        JsonAssert::assertJsonMatchesSchema($json, dirname(__DIR__, 3) . '/Resources/errors.schema.json');
+        JsonAssert::assertJsonMatchesSchema(json_decode($json), dirname(__DIR__, 3) . '/Resources/errors.schema.json');
 
         self::assertJsonStringEqualsJsonString(
             <<<'OUTPUT'