Fix example in Javadoc for MultipartBodyBuilder

See gh-30593

spring-web/src/main/java/org/springframework/http/client/MultipartBodyBuilder.java

@@ -76,7 +76,7 @@ import org.springframework.util.MultiValueMap;
  *
  * Mono<Void> result = webClient.post()
  *     .uri("...")
- *     .body(multipartBody)
+ *     .body(BodyInserters.fromMultipartData(multipartBody))
  *     .retrieve()
  *     .bodyToMono(Void.class)
  * </pre>