pretty-options.txt: --notes accepts a ref instead of treeish

pretty-options.txt: --notes accepts a ref instead of treeish

Although `--notes=` accepts and handles a tree-ish just fine, it isn't a
common use-case for users to pass in bare trees. By having "treeish", it
makes it harder to click in users' minds that the argument here is the
same type as the `notes.displayRef` configuration variable, for example.

Change `treeish` to `ref` so that it will be easier for users to make
this connection.

Note that we don't completely lose the notion that `--notes=` can accept
a tree-ish. In git-notes.txt, we have

	It is also permitted for a notes ref to point directly to a tree
	object, in which case the history of the notes can be read with
	`git log -p -g <refname>`.

which means that a hardcore user who wants to take advantage of this
obscure use-case will be able to infer the connection and not be
completely left in the dark.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>