fix(example): Use correct type in comment

Fixes #2631

examples/15_custom_validator.rs

@@ -2,8 +2,8 @@ use clap::{App, Arg};
 
 fn main() {
     // You can define a function (or a closure) to use as a validator to argument values. The
-    // function must accept a String and return Result<(), String> where Err(String) is the message
-    // displayed to the user.
+    // function must accept a `&str` and return `Result<(), String>` where `Err(String)` is the
+    // message displayed to the user.
 
     let matches = App::new("myapp")
         // Application logic goes here...