doc: fix non-possessive "'s"

doc: fix non-possessive "'s"

Currently we use "Section's" as the plural of the debug/elf Section
struct. Change this to "Sections" because it's not possessive and
doesn't seem to fall in to any special cases were the apostrophe is
acceptable.

Change-Id: Id5d3abbd748502a67ead3f483182ee7729db94a2
Reviewed-on: https://go-review.googlesource.com/19505
Reviewed-by: Russ Cox <rsc@golang.org>