Update --depth help messages

Co-Authored-By: jorpic <jorpic@gmail.com>

src/app.rs

@@ -72,7 +72,7 @@ pub fn build() -> App<'static, 'static> {
             Arg::with_name("depth")
                 .long("depth")
                 .takes_value(true)
-                .value_name("NUM")
+                .value_name("num")
                 .help("Stop recursing into directories after reaching specified depth"),
         )
         .arg(

src/flags.rs

@@ -40,7 +40,7 @@ impl Flags {
                 Ok(val) => val,
                 Err(_) => {
                     return Err(Error::with_description(
-                        "The argument '--depth' requires a valid number",
+                        "The argument '--depth' requires a valid positive number",
                         ErrorKind::ValueValidation,
                     ))
                 }