Fix typo in assertion message.

Corrected the assertion message from "Sample must not be null" to "Example must not be null" for clarity and consistency.

Closes #3565

Author: Seol-JY

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/SimpleJpaRepository.java

@@ -590,7 +590,7 @@ public <S extends T> Page<S> findAll(Example<S> example, Pageable pageable) {
 	@Override
 	public <S extends T, R> R findBy(Example<S> example, Function<FetchableFluentQuery<S>, R> queryFunction) {
 
-		Assert.notNull(example, "Sample must not be null");
+		Assert.notNull(example, "Example must not be null");
 		Assert.notNull(queryFunction, "Query function must not be null");
 
 		ExampleSpecification<S> spec = new ExampleSpecification<>(example, escapeCharacter);