doc: Write "file system" instead of "partition" as appopriate.

* doc/guix.texi (Preparing for Installation): Write "file system" instead
of "partition" as appopriate.

doc/guix.texi

@@ -8072,7 +8072,7 @@ types.}.  For the ESP, if you have one and assuming it is
 mkfs.fat -F32 /dev/sda2
 @end example
-Preferably, assign partitions a label so that you can easily and
+Preferably, assign file systems a label so that you can easily and
 reliably refer to them in @code{file-system} declarations (@pxref{File
 Systems}).  This is typically done using the @code{-L} option of
 @command{mkfs.ext4} and related commands.  So, assuming the target root
@@ -8097,9 +8097,9 @@ cryptsetup open --type luks /dev/sda1 my-partition
 mkfs.ext4 -L my-root /dev/mapper/my-partition
 @end example
-Once that is done, mount the target root partition under @file{/mnt}
+Once that is done, mount the target file system under @file{/mnt}
 with a command like (again, assuming @code{my-root} is the label of the
-root partition):
+root file system):
 @example
 mount LABEL=my-root /mnt