merge-tree: fix argument type of the `--merge-base` option

merge-tree: fix argument type of the `--merge-base` option

In 5f43cf5b2e4 (merge-tree: accept 3 trees as arguments, 2024-01-28), I
taught `git merge-tree` to perform three-way merges on trees. This
commit even changed the manual page to state that the `--merge-base`
option takes a tree-ish rather than requiring a commit.

But I forgot to adjust the in-program help text. This patch fixes that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>