ls(1): Refine the HISTORY within the manual page.

A simple find command appeared in Version 1 AT&T UNIX and was removed in
Version 3 AT&T UNIX. It was rewritten for Version 5 AT&T UNIX and later
be enhanced for the Programmer's Workbench (PWB). These changes were
later incorporated in AT&T UNIX v7.

Reviewed by: imp
MFC after: 1 week