filter-branch.sh: de-dent usage string

filter-branch.sh: de-dent usage string

"Usage: git filter-branch " that is prefixed to the first line is 25
columns long, so the "[--index-filter ..." on the second line would not
align with "[--env-filter ..." on the first line to begin with. If the
second and subsequent lines do not aim to align with anything on the
first line, it is just fine to indent them with a single HT.

Signed-off-by: Junio C Hamano <gitster@pobox.com>