docs: fix example description about constructor

Author: jonhteper

README.md

@@ -14,7 +14,7 @@ use non_empty_string::NonEmptyString;
 let s = "A string with a length".to_owned();
 assert!(NonEmptyString::new(s).is_ok());
 
-// But constructing it from a non-zero-length String results in an `Err`, where we get the `String` back that we passed in.
+// But constructing it from a zero-length String results in an `Err`, where we get the `String` back that we passed in.
 let empty = "".to_owned();
 let result = NonEmptyString::new(empty);
 assert!(result.is_err());