filter-branch: fix remnants of old syntax in documentation
Some time ago, filter-branch's syntax changed so that more than one
ref can be rewritten at the same time. This involved the removal of
the ref name for the result; instead, the refs are rewritten in-place.
This updates the last leftovers in the documentation to reflect the
new behavior.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>