chore: Work around HTTP 500 at ceph.io

Apparently, accessing https://ceph.io/ (which should cause a redirect
to https://ceph.io/en) occasionally produces an Internal Server Error,
which directly linking to https://ceph.io/en does not cause. Thus,
link to the latter directly.

Author: fghaas

docs/background/disaster-recovery.md

@@ -22,7 +22,7 @@ background, and why you should consider enabling it.
 ## What it is
 
 The *Disaster Recovery* (DR) feature is available via the {{gui}} and
-applies to servers and volumes that use our [Ceph](https://ceph.io/en/)
+applies to servers and volumes that use our [Ceph](https://ceph.io)
 backend. That would be **all** servers but the ones of the `s`
 [flavor](../../reference/flavors/#compute-tiers).