docs: replace Content-type with Content-Type

Author: kenjis

user_guide_src/source/concepts/http/002.php

@@ -6,7 +6,7 @@
 
 $response->setStatusCode(Response::HTTP_OK);
 $response->setBody($output);
-$response->setHeader('Content-type', 'text/html');
+$response->setHeader('Content-Type', 'text/html');
 $response->noCache();
 
 // Sends the output to the browser