fix: add comment to callout

Author: timClicks

ch3/fileresult/src/main.rs

@@ -37,7 +37,7 @@ impl File {
 }
 
 fn open(f: File) -> Result<File, String> {
-  if one_in(10_000) {                              <7>
+  if one_in(10_000) {                              // <7>
     let err_msg = String::from("Permission denied");
     return Err(err_msg);
   }
@@ -67,4 +67,4 @@ fn main() {
   println!("{:?}", f4);
   println!("{} is {} bytes long", &f4.name, f4_length);
   println!("{}", text);
-}
+}