Define rclone's remote where it is first used

The documents use "remote key" for something else, so "remote" as
rclone's noun is now defined on first use: a remote is a storage service
set up in rclone. Changed in `README.md` (the destination bullet),
`docs/REPOSTRUCTURE.md` (the opening) and `config.example.yml` (the
`storage_url` comment).

Model: fable-5-1

docs/REPOSTRUCTURE.md

@@ -4,8 +4,8 @@ This document describes what `vaultik` stores at the destination. It covers the
 layout, what the layout protects, and the risks it accepts.
 
 The destination is the place the backups are stored. It is an S3 bucket and
-prefix, a local directory or a remote of [rclone](https://rclone.org/). The
-layout is the same on all three.
+prefix, a local directory or an [rclone](https://rclone.org/) remote. A remote
+is a storage service set up in rclone. The layout is the same on all three.
 
 ## Terms