Doc: fix links using url for internal doc

Documentation/op-guide/failures.md

@@ -40,5 +40,5 @@ A cluster bootstrap is only successful if all required members successfully star
 
 Of course, it is possible to recover a failed bootstrapped cluster like recovering a running cluster. However, it almost always takes more time and resources to recover that cluster than bootstrapping a new one, since there is no data to recover.
 
-[backup]: https://github.com/coreos/etcd/blob/master/Documentation/op-guide/maintenance.md#snapshot-backup
-[unrecoverable]: https://github.com/coreos/etcd/blob/master/Documentation/op-guide/recovery.md#disaster-recovery
+[backup]: maintenance.md#snapshot-backup
+[unrecoverable]: recovery.md#disaster-recovery