fix: response with correct HTTP/1.x Message format

The correct `HTTP/1.x Message`'s headers are followed after the `start-line` which describing the requests to be implemented. After the `headers` is a `blank line` indicating all `meta-information` for the request has been sent. Then the `optional body` is followed.

src/ch20-01-single-threaded.md

@@ -371,9 +371,9 @@ fn handle_connection(mut stream: TcpStream) {
     // --snip--
 
     let (status_line, filename) = if buffer.starts_with(get) {
-        ("HTTP/1.1 200 OK\r\n\r\n", "hello.html")
+        ("HTTP/1.1 200 OK", "hello.html")
     } else {
-        ("HTTP/1.1 404 NOT FOUND\r\n\r\n", "404.html")
+        ("HTTP/1.1 404 NOT FOUND", "404.html")
     };
 
     let contents = fs::read_to_string(filename).unwrap();