doc: Unconfuse file systems and partitions.

Partitions can have their own labels and UUIDs (e.g. under GPT) that are
unrelated to those of the contained file system.  Confusing the two ends
poorly.

* doc/guix.scm (File Systems, Proceeding with the Installation)
(Using the Configuration System, Initial RAM Disk): Substitute `file
system' for `partition' when talking about labels and UUIDs.