修复示例代码中的语法错误

src/ch20-01-single-threaded.md

@@ -315,7 +315,11 @@ fn handle_connection(mut stream: TcpStream) {
     let status_line = "HTTP/1.1 404 NOT FOUND\r\n\r\n";
     let contents = fs::read_to_string("404.html").unwrap();
 
-    let response = format!("{}\r\nContent-Length: {}\r\n\r\n{}", status_line, contents);
+    let response = format!("{}\r\nContent-Length: {}\r\n\r\n{}",
+        status_line,
+        contents.len(),
+        contents
+    );
 
     stream.write(response.as_bytes()).unwrap();
     stream.flush().unwrap();
@@ -374,7 +378,11 @@ fn handle_connection(mut stream: TcpStream) {
 
     let contents = fs::read_to_string(filename).unwrap();
 
-    let response = format!("{}\r\nContent-Length: {}\r\n\r\n{}", status_line, contents);
+    let response = format!("{}\r\nContent-Length: {}\r\n\r\n{}",
+        status_line,
+        contents.len(),
+        contents
+    );
 
     stream.write(response.as_bytes()).unwrap();
     stream.flush().unwrap();