DATAMONGO-2118 - Polishing.

Fix typo in reactive repositories reference documentation.

Original pull request: spring-projects#611.

Author: mp911de

src/main/asciidoc/reference/reactive-mongo-repositories.adoc

@@ -47,7 +47,7 @@ Note that the entity defined in the preceding example has a property named `id`
 ====
 [source]
 ----
-public interface ReactivePersonRepository extends ReactiveSortingRepository<Person, Long> {
+public interface ReactivePersonRepository extends ReactiveSortingRepository<Person, String> {
 
   Flux<Person> findByFirstname(String firstname);                                   <1>