doc/architecture: repair RBD sentence

doc/architecture: repair RBD sentence

Improve an ambiguous sentence in doc/architecture.rst.

The problem presented by the original sentence is that the phrasal verb
"to provide with" is implicated in one of its possible readings.
Interpreted in that way, the sentence seems to express the incorrect
idea that RBD furnishes block devices with snapshotting and cloning, as
though snapshotting and cloning are being delivered to the block
devices. In fact, snapshotting and cloning are just features of RBD, and
are features that are described on this page:
https://docs.ceph.com/en/quincy/rbd/rbd-snapshot/.

Signed-off-by: Zac Dover <zac.dover@proton.me>