Add example usage for formatting a floppy disk. Adding a more self

Add example usage for formatting a floppy disk.  Adding a more self
contained example here in the fdformat man page will allow us to
modernize and streamline the FreeBSD Handbook by cutting out some of
this legacy material.

While here, address some other minor grammatical nits in this man page.

Reviewed by:	bcr (mentor)
Approved by:	bcr (mentor)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D24971