Update test name in WebFluxTagsTests

See gh-19976

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/web/reactive/server/WebFluxTagsTests.java

@@ -113,7 +113,7 @@ class WebFluxTagsTests {
 	}
 
 	@Test
-	void outcomeTagIsUnknownWhenResponseStatusIsNull() {
+	void outcomeTagIsSuccessWhenResponseStatusIsNull() {
 		this.exchange.getResponse().setStatusCode(null);
 		Tag tag = WebFluxTags.outcome(this.exchange);
 		assertThat(tag.getValue()).isEqualTo("SUCCESS");