buffers-menu-show-directories is a variable defined in `menu-bar.el'.
Its value is

If non-nil, show directories in the Buffers menu for buffers that have them.
The special value `unless-uniquify' means that directories will be shown
unless `uniquify-buffer-name-style' is non-nil (in which case, buffer
names should include enough of a buffer's directory to distinguish it
from other buffers).

Setting this variable directly does not take effect until next time the
Buffers menu is regenerated.

You can customize this variable.