fix: Correct UserMessage example to use builder

The documentation example for UserMessage incorrectly used a private
constructor. This commit updates the example to use the public
`builder()` method, making the code functional.

Signed-off-by: MoGreene <[email protected]>

Author: Mo-Greene

spring-ai-docs/src/main/antora/modules/ROOT/pages/api/chat/google-genai-chat.adoc

@@ -209,8 +209,10 @@ Below is a simple code example extracted from https://github.com/spring-projects
 ----
 byte[] data = new ClassPathResource("/vertex-test.png").getContentAsByteArray();
 
-var userMessage = new UserMessage("Explain what do you see on this picture?",
-        List.of(new Media(MimeTypeUtils.IMAGE_PNG, this.data)));
+var userMessage = UserMessage.builder()
+			.text("Explain what do you see o this picture?")
+			.media(List.of(new Media(MimeTypeUtils.IMAGE_PNG, data)))
+			.build();
 
 ChatResponse response = chatModel.call(new Prompt(List.of(this.userMessage)));
 ----
@@ -224,9 +226,10 @@ Use the `application/pdf` media type to attach a PDF file to the message:
 ----
 var pdfData = new ClassPathResource("/spring-ai-reference-overview.pdf");
 
-var userMessage = new UserMessage(
-        "You are a very professional document summarization specialist. Please summarize the given document.",
-        List.of(new Media(new MimeType("application", "pdf"), pdfData)));
+var userMessage = UserMessage.builder()
+			.text("You are a very professional document summarization specialist. Please summarize the given document.")
+			.media(List.of(new Media(new MimeType("application", "pdf"), pdfData)))
+			.build();
 
 var response = this.chatModel.call(new Prompt(List.of(userMessage)));
 ----