git-rebase.txt: fix description list separator

git-rebase.txt: fix description list separator

We don't give a "::" for the list separator, but just a single ":". This
ends up rendering literally, "--apply: Use applying strategies ...". As
a follow-on error, the list continuation, "+", also ends up rendering
literally (because we don't have a list).

This was introduced in 52eb738d6b ("rebase: add an --am option",
2020-02-15) and survived the rename in 10cdb9f38a ("rebase: rename the
two primary rebase backends", 2020-02-15).

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>