chore: refine sample code.

Author: ya7010

serde_valid/README.md

@@ -82,7 +82,7 @@ struct Data {
 let err = Data::from_json_value(json!({ "val": 123 })).unwrap_err();
 
 assert_eq!(
-    err.as_validation_errors().unwrap().to_string(),
+    err.to_string(),
     json!({
         "errors": [],
         "properties": {

serde_valid/src/lib.rs

@@ -82,7 +82,7 @@
 //! let err = Data::from_json_value(json!({ "val": 123 })).unwrap_err();
 //!
 //! assert_eq!(
-//!     err.as_validation_errors().unwrap().to_string(),
+//!     err.to_string(),
 //!     json!({
 //!         "errors": [],
 //!         "properties": {