Fix broken links in reference documentation

The two links below should likely point to `snapshot` instead of `libs-snapshot`
```
403 FORBIDDEN: https://repo.spring.io/ui/repos/tree/General/libs-snapshot/org/springframework/data/spring-data-couchbase
403 FORBIDDEN: https://repo.spring.io/ui/repos/tree/General/libs-milestone/org/springframework/data/spring-data-couchbase
```