doc: ext3: update documentation with barrier=1 default

Commit 00eacd66

 ("ext3: make ext3 mount default to barrier=1") changed
the default barrier mount option for ext3.  The documentation needs to
be updated, so this patch does that.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>