bup-save.md: fix --graft documentation bug; use "chroot" consistently.
bup-save.md: fix --graft documentation bug; use "chroot" consistently.

Change mixture of "chroot" and "chroots" in examples to "chroot".

Thanks to "jon.seymour" <jon.seymour@gmail.com> for the report.

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

ToxicFrog/bup,ToxicFrog/bup,ToxicFrog/bup,gevaerts/bup,ToxicFrog/bup,jbaber/bup,tjanez/bup,jbaber/bup,gevaerts/bup,jbaber/bup,jbaber/bup,jbaber/bup,gevaerts/bup,pombredanne/bup,tjanez/bup,mhoeher/bup,mhoeher/bup,mhoeher/bup,mhoeher/bup,gevaerts/bup,pombredanne/bup,tjanez/bup,jbaber/bup,pombredanne/bup,gevaerts/bup,pombredanne/bup,mhoeher/bup,pombredanne/bup,tjanez/bup,tjanez/bup,pombredanne/bup,mhoeher/bup,tjanez/bup,ToxicFrog/bup