Update auditing reference documentation regarding changes in API of AuditorAware [DATACMNS-1270]

[spring-projects-issues]

Oliver Drotbohm opened DATACMNS-1270 and commented

The return type of AuditorAware.getCurrentAuditor() has changed to Optional<T> in Spring Data 2.0

---

Reference URL: https://docs.spring.io/spring-data/jpa/docs/current/reference/html/index.html#auditing.auditor-aware

[christophstrobl]

fixed via #321