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/19505Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.6.html

@@ -569,7 +569,7 @@ The <a href="/pkg/debug/elf/"><code>debug/elf</code></a> package
 adds support for general compressed ELF sections.
 User code needs no updating: the sections are decompressed automatically when read.
 However, compressed
-<a href="/pkg/debug/elf/#Section"><code>Section</code></a>'s do not support random access:
+<a href="/pkg/debug/elf/#Section"><code>Sections</code></a> do not support random access:
 they have a nil <code>ReaderAt</code> field.
 </li>