Some minor tweaks to arch(7).

• Drop mention of _LP64. FreeBSD's source generally uses LP64 instead of _LP64, and the relevant macros are better covered in the "Predefined Macros" section.
• Fix a noun/verb disagreement.

Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22975