Update spring-cloud-stream.adoc

Remove `@RunWith(SpringRunner.class)` from sample code in Spring Integration Test Binder section:
1. It is a JUnit4 annotation
2. `@SpringBootTest` composes the JUnit5 counterpart (`@ExtendWith({SpringExtension.class})`)

Resolves #2537

docs/src/main/asciidoc/spring-cloud-stream.adoc

@@ -2917,7 +2917,6 @@ Now you can test your microservice as a simple unit test
 [source,java]
 ----
 @SpringBootTest
-@RunWith(SpringRunner.class)
 public class SampleStreamTests {
 
 	@Autowired