filter-branch: fix remnants of old syntax in documentation

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>