pxeboot(8): remove antiquated information

While I was there:

• Fix some typos
• Fix an excessive argument "indent" reported by mandoc -Tlint
• Replace a dead link with the one suggested by https://www.uefi.org/uefi

Submitted by: linimon (in part)
Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D27774

(cherry picked from commit c90fb7acf099f1dc982f8d6087aa92809ae43727)