Use `--one-file-system` instead of `--do-not-cross-mountpoints` in docs.

Related to #296 in which support for `--do-not-cross-mountpoints` has
been deprecated.

docs/usage.rst

@@ -251,7 +251,7 @@ Examples
     # Backup the root filesystem into an archive named "root-YYYY-MM-DD"
     # use zlib compression (good, but slow) - default is no compression
     NAME="root-`date +%Y-%m-%d`"
-    $ borg create -C zlib,6 /mnt/backup::$NAME / --do-not-cross-mountpoints
+    $ borg create -C zlib,6 /mnt/backup::$NAME / --one-file-system
 
     # Make a big effort in fine granular deduplication (big chunk management
     # overhead, needs a lot of RAM and disk space, see formula in internals
@@ -381,7 +381,7 @@ Examples
     Hostname: myhostname
     Username: root
     Time: Fri Aug  2 15:18:17 2013
-    Command line: /usr/bin/borg create --stats -C zlib,6 /mnt/backup::root-2013-08-02 / --do-not-cross-mountpoints
+    Command line: /usr/bin/borg create --stats -C zlib,6 /mnt/backup::root-2013-08-02 / --one-file-system
     Number of files: 147429
     Original size: 5344169493 (4.98 GB)
     Compressed size: 1748189642 (1.63 GB)