Refine FactCheckingEvaluator prompt to specify yes/no answer.

Author: habuma

spring-ai-core/src/main/java/org/springframework/ai/evaluation/FactCheckingEvaluator.java

@@ -64,6 +64,9 @@
 public class FactCheckingEvaluator implements Evaluator {
 
 	private static final String DEFAULT_EVALUATION_PROMPT_TEXT = """
+				Evaluate whether or not the following claim is supported by the provided document.
+				Respond with "yes" if the claim is supported, or "no" if it is not.
+
 				Document: \\n {document}\\n
 				Claim: \\n {claim}
 			""";