fix: readme Msg (#184)

## AgentScope-Java Version

[The version of AgentScope-Java you are working on, e.g. 1.0.2, check
your pom.xml dependency version or run `mvn dependency:tree | grep
agentscope-parent:pom`(only mac/linux)]

## Description

1.0.2 version，Msg.class No userMsg method
<img width="848" height="415" alt="image"
src="https://github.com/user-attachments/assets/fb257ef2-13e9-45a1-86ce-0ea086fb22de"
/>

## Checklist

Please check the following items before code is ready to be reviewed.

- [ ]  Code has been formatted with `mvn spotless:apply`
- [ ]  All tests are passing (`mvn test`)
- [ ]  Javadoc comments are complete and follow project conventions
- [ ] Related documentation has been updated (e.g. links, examples,
etc.)
- [ ]  Code is ready for review

Author: GTyingzi

README.md

@@ -90,7 +90,9 @@ ReActAgent agent = ReActAgent.builder()
         .build())
     .build();
 
-Msg response = agent.call(Msg.userMsg("Hello!")).block();
+Msg response = agent.call(Msg.builder()
+        .textContent("Hello!")
+        .build()).block();
 System.out.println(response.getTextContent());
 ```
 

README_zh.md

@@ -90,7 +90,9 @@ ReActAgent agent = ReActAgent.builder()
         .build())
     .build();
 
-Msg response = agent.call(Msg.userMsg("Hello!")).block();
+Msg response = agent.call(Msg.builder()
+        .textContent("Hello!")
+        .build()).block();
 System.out.println(response.getTextContent());
 ```