editorial update

docs/content_ja.md

@@ -774,7 +774,7 @@ println!("{}", label);
 let hello = "Hello"; 
 let world = "world!";
 let hello = hello.to_string();
-let buffer = new String();
+let mut buffer = new String();
 buffer = buffer + &hello + world;
 println!("{}", buffer);
 ~~~